1+ package com .leetcode .trees ;
2+ 3+ import java .util .ArrayList ;
4+ import java .util .List ;
5+ 6+ import static org .junit .jupiter .api .Assertions .assertEquals ;
7+ 8+ /**
9+ * Level: Medium
10+ * Problem Link: https://leetcode.com/problems/find-leaves-of-binary-tree/
11+ * Problem Description:
12+ * Given a binary tree, collect a tree's nodes as if you were doing this: Collect and remove all leaves, repeat
13+ * until the tree is empty.
14+ *
15+ * Example:
16+ * Input: [1,2,3,4,5]
17+ *
18+ * 1
19+ * / \
20+ * 2 3
21+ * / \
22+ * 4 5
23+ *
24+ * Output: [[4,5,3],[2],[1]]
25+ *
26+ * Explanation:
27+ * 1. Removing the leaves [4,5,3] would result in this tree:
28+ * 1
29+ * /
30+ * 2
31+ *
32+ * 2. Now removing the leaf [2] would result in this tree:
33+ * 1
34+ *
35+ * 3. Now removing the leaf [1] would result in the empty tree:
36+ * []
37+ *
38+ * @author rampatra
39+ * @since 2019年08月01日
40+ */
41+ public class LeavesOfBinaryTree {
42+ 43+ /**
44+ * THe idea is to perform a DFS and backtrack. While backtracking, check the height of the node and insert
45+ * the node into the list indexed by their heights.
46+ * Time Complexity:
47+ * Space Complexity:
48+ * Runtime: <a href=""></a>.
49+ *
50+ * @param root
51+ * @return
52+ */
53+ public static List <List <TreeNode >> findLeavesOfBinaryTree (TreeNode root ) {
54+ List <List <TreeNode >> levels = new ArrayList <>();
55+ findLeavesOfBinaryTree (root , levels );
56+ return levels ;
57+ }
58+ 59+ private static int findLeavesOfBinaryTree (TreeNode root , List <List <TreeNode >> levels ) {
60+ if (root == null ) return -1 ;
61+ 62+ int leftHeight = findLeavesOfBinaryTree (root .left , levels );
63+ int rightHeight = findLeavesOfBinaryTree (root .right , levels );
64+ int height = Math .max (leftHeight , rightHeight ) + 1 ;
65+ 66+ if (height >= levels .size ()) {
67+ levels .add (height , new ArrayList <>());
68+ }
69+ levels .get (height ).add (root );
70+ 71+ return height ;
72+ }
73+ 74+ public static void main (String [] args ) {
75+ /*
76+ BST looks like:
77+
78+ 4
79+ / \
80+ 1 7
81+ / \ \
82+ 3 8 20
83+ / \
84+ 2 6
85+ */
86+ TreeNode root = new TreeNode (4 );
87+ root .left = new TreeNode (1 );
88+ root .right = new TreeNode (7 );
89+ root .left .left = new TreeNode (3 );
90+ root .left .right = new TreeNode (8 );
91+ root .left .left .left = new TreeNode (2 );
92+ root .left .left .right = new TreeNode (6 );
93+ root .right .right = new TreeNode (20 );
94+ 95+ assertEquals ("[[2, 6, 8, 20], [3, 7], [1], [4]]" , findLeavesOfBinaryTree (root ).toString ());
96+ }
97+ }
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