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Commit eb42850

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committed
20190515
1 parent c86a4ba commit eb42850

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5 files changed

+178
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5 files changed

+178
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‎code/lc559.java

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package code;
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import java.util.List;
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/*
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* 559. Maximum Depth of N-ary Tree
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* 题意:多叉树的最大深度
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* 难度:Easy
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* 分类:Tree, Depth-first Search, Breadth-first Search
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* 思路:BFS 也能做和 lc104思路一样
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* Tips:lc104, lc111
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*/
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public class lc559 {
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class Node {
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public int val;
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public List<Node> children;
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public Node() {}
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public Node(int _val,List<Node> _children) {
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val = _val;
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children = _children;
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}
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}
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public int maxDepth(Node root) {
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if(root==null) return 0;
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int res = 0;
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for (Node nd : root.children) {
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res = Math.max(maxDepth(nd), res);
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}
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return res+1;
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}
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}

‎code/lc589.java

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package code;
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import java.util.ArrayList;
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import java.util.List;
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/*
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* 589. N-ary Tree Preorder Traversal
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* 题意:多叉树先序遍历
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* 难度:Easy
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* 分类:Tree
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* 思路:
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* Tips:
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*/
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public class lc589 {
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class Node {
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public int val;
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public List<Node> children;
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public Node() {}
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public Node(int _val,List<Node> _children) {
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val = _val;
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children = _children;
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}
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}
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List<Integer> res;
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public List<Integer> preorder(Node root) {
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res = new ArrayList<>();
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helper(root);
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return res;
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}
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public void helper(Node root){
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if(root==null) return;
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res.add(root.val);
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for(Node nd:root.children){
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helper(nd);
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}
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}
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}

‎code/lc590.java

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package code;
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import java.util.ArrayList;
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import java.util.List;
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public class lc590 {
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class Node {
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public int val;
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public List<Node> children;
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public Node() {}
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public Node(int _val,List<Node> _children) {
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val = _val;
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children = _children;
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}
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}
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List<Integer> res;
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public List<Integer> postorder(Node root) {
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res = new ArrayList<>();
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helper(root);
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return res;
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}
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public void helper(Node root){
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if(root==null) return;
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for(Node nd:root.children){
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helper(nd);
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}
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res.add(root.val);
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}
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}

‎code/lc834.java

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package code;
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import java.util.HashMap;
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import java.util.HashSet;
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/*
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* 834. Sum of Distances in Tree
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* 题意:求树中每个节点的值,到其他节点的距离和
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* 难度:Hard
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* 分类:Tree, Depth-first Search
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* 思路:真的难,我是做不出来
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* 两次遍历,第一次后续遍历,计算出每个节点为根的树,有几个孩子节点,并计算出root的结果
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* 根据前一步的计算结果,开始先序遍历,一步步计算出其他节点的结果
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* res[node] = res[parent]-2*count[node]+count.length;
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* https://leetcode.com/problems/sum-of-distances-in-tree/discuss/130583/C%2B%2BJavaPython-Pre-order-and-Post-order-DFS-O(N)
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* https://leetcode.com/problems/sum-of-distances-in-tree/discuss/130567/Two-traversals-O(N)-python-solution-with-Explanation
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* Tips:
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*/
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public class lc834 {
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public static void main(String[] args) {
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int[][] arr = {{0,1},{0,2},{2,3},{2,4},{2,5}};
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sumOfDistancesInTree(6, arr);
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}
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static int[] count;
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static int[] res;
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static HashMap<Integer, HashSet<Integer>> hm = new HashMap();
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public static int[] sumOfDistancesInTree(int N, int[][] edges) {
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if(edges.length==0) return new int[]{0};
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count = new int[N]; //记录n为跟的树,下边有多少个节点
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res = new int[N];
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for (int i = 0; i < edges.length ; i++) { //双向都添加,遍历的时候判断一下,因为并不一定0就是根
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HashSet<Integer> hs = hm.getOrDefault(edges[i][0], new HashSet());
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hs.add(edges[i][1]);
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hm.put(edges[i][0], hs);
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hs = hm.getOrDefault(edges[i][1], new HashSet());
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hs.add(edges[i][0]);
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hm.put(edges[i][1], hs);
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}
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helper1(0, -1);
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helper2(0, -1);
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return res;
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}
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public static int helper1(int node, int parent){ //后序遍历,求root对应的结果,计算每个节点的count
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HashSet<Integer> hs = hm.getOrDefault(node, new HashSet());
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for (Integer i:hs) {
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if(i==parent) continue; //是parent的话直接略过
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count[node] += helper1(i, node);
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res[node] += res[i] + count[i];
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}
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count[node]++;
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return count[node];
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}
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public static void helper2(int node, int parent){ //先序遍历,求结果
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HashSet<Integer> hs = hm.getOrDefault(node, new HashSet());
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if(node!=0)
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res[node] = res[parent]-2*count[node]+count.length; //转移计算
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for (Integer i:hs) {
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if(i==parent) continue; //是parent的话直接略过
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helper2(i, node);
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}
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}
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}

‎readme.md

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| 494 [Java](./code/lc494.java)
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| 538 [Java](./code/lc538.java)
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| 543 [Java](./code/lc543.java)
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| 559 [Java](./code/lc559.java)
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| 560 [Java](./code/lc543.java)
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| 572 [Java](./code/lc572.java)
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| 581 [Java](./code/lc581.java)
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| 589 [Java](./code/lc589.java)
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| 590 [Java](./code/lc590.java)
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| 617 [Java](./code/lc617.java)
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| 621 [Java](./code/lc621.java)
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| 647 [Java](./code/lc647.java)
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| 685 [Java](./code/lc685.java)
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| 714 [Java](./code/lc714.java)
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| 746 [Java](./code/lc746.java)
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| 771 [Java](./code/lc771.java)
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| 834 [Java](./code/lc834.java)
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| 877 [Java](./code/lc877.java)
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| 921 [Java](./code/lc921.java)
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| 922 [Java](./code/lc922.java)

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