Commit 42f7ad6

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20200924

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	`1`	`+/*`
	`2`	`+ * @lc app=leetcode id=14 lang=java`
	`3`	`+ *`
	`4`	`+ * [14] Longest Common Prefix`
	`5`	`+ */`
	`6`	`+`
	`7`	`+// @lc code=start`
	`8`	`+class Solution {`
	`9`	`+ public String longestCommonPrefix(String[] strs) {`
	`10`	`+ if (strs == null \|\| strs.length == 0) return "";`
	`11`	`+ for (int i = 0; i < strs[0].length(); ++i) {`
	`12`	`+ char c = strs[0].charAt(i);`
	`13`	`+ for (int j = 1; j < strs.length; ++j) {`
	`14`	`+ if (i == strs[j].length() \|\| strs[j].charAt(i) != c) {`
	`15`	`+ return strs[0].substring(0, i);`
	`16`	`+ }`
	`17`	`+ }`
	`18`	`+ }`
	`19`	`+ return strs[0];`
	`20`	`+ }`
	`21`	`+}`
	`22`	`+// @lc code=end`
	`23`	`+`

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	`1`	`+/*`
	`2`	`+ * @lc app=leetcode id=818 lang=java`
	`3`	`+ *`
	`4`	`+ * [818] Race Car`
	`5`	`+ */`
	`6`	`+`
	`7`	`+// @lc code=start`
	`8`	`+class Solution {`
	`9`	`+ /*`
	`10`	`+ dp: 子问题: 到某一个位置之后restart(掉头,速度重置)`
	`11`	`+ dp[t][d]: 从当前位置走到target并面向d(left = 0, right = 1)的最小action`
	`12`	`+ dp[t][d] = min(dp[i][0] + 2 + dp[t - i][d], (2:掉头再掉头)`
	`13`	`+ dp[i][1] + 1 + dp[t - i][d])`
	`14`	`+ time: O(n^2)`
	`15`	`+ space: O(n)`
	`16`	`+ */`
	`17`	`+ public int racecar(int target) {`
	`18`	`+ if (target == 0) return 0;`
	`19`	`+ int[][] dp = new int[target + 1][2];`
	`20`	`+ for (int t = 1; t <= target; ++t) {`
	`21`	`+ int n = (int)Math.ceil(Math.log(t + 1) / Math.log(2));`
	`22`	`+ if ((1 << n) == t + 1) {`
	`23`	`+ // 一直加速可以到`
	`24`	`+ dp[t][0] = n;`
	`25`	`+ dp[t][1] = n + 1;`
	`26`	`+ continue;`
	`27`	`+ }`
	`28`	`+ // 需要倒车`
	`29`	`+ int l = (1 << n) - 1 - t;`
	`30`	`+ dp[t][0] = n + 1 + Math.min(dp[l][1], dp[l][0] + 1);`
	`31`	`+ dp[t][1] = n + 1 + Math.min(dp[l][0], dp[l][1] + 1);`
	`32`	`+ // 遍历所有可能的restart`
	`33`	`+ for (int i = 1; i < t; ++i) {`
	`34`	`+ for (int d = 0; d <= 1; ++d) {`
	`35`	`+ dp[t][d] = Math.min(dp[t][d], Math.min(`
	`36`	`+ dp[i][0] + 2 + dp[t - i][d],`
	`37`	`+ dp[i][1] + 1 + dp[t - i][d]));`
	`38`	`+ }`
	`39`	`+ }`
	`40`	`+ }`
	`41`	`+ return Math.min(dp[target][0], dp[target][1]);`
	`42`	`+ }`
	`43`	`+}`
	`44`	`+// @lc code=end`
	`45`	`+`

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	`1`	`+/*`
	`2`	`+ * @lc app=leetcode id=818 lang=java`
	`3`	`+ *`
	`4`	`+ * [818] Race Car`
	`5`	`+ */`
	`6`	`+`
	`7`	`+// @lc code=start`
	`8`	`+class Solution {`
	`9`	`+ /*`
	`10`	`+ 将A和R看作每一步的两个状态,用BFS来找最短路径`
	`11`	`+ time: O(2^n)`
	`12`	`+ space: O(2^n)`
	`13`	`+ */`
	`14`	`+ public int racecar(int target) {`
	`15`	`+ Queue<int[]> q = new LinkedList<>();`
	`16`	`+ q.offer(new int[]{0, 1});`
	`17`	`+ Set<String> visited = new HashSet<>();`
	`18`	`+ visited.add("0_1");`
	`19`	`+ int step = 0;`
	`20`	`+ while (!q.isEmpty()) {`
	`21`	`+ int size = q.size();`
	`22`	`+ while (size-- > 0) {`
	`23`	`+ int[] cur = q.poll();`
	`24`	`+ int pos = cur[0];`
	`25`	`+ int speed = cur[1];`
	`26`	`+ if (pos == target) return step;`
	`27`	`+ // A`
	`28`	`+ int pos1 = pos + speed;`
	`29`	`+ int speed1 = speed * 2;`
	`30`	`+ q.offer(new int[]{pos1, speed1}); // 加进visited数组会超时`
	`31`	`+ // R`
	`32`	`+ int speed2 = speed > 0 ? -1 : 1;`
	`33`	`+ if (visited.add(pos + "_" + speed2)) {`
	`34`	`+ q.offer(new int[]{pos, speed2});`
	`35`	`+ }`
	`36`	`+ }`
	`37`	`+ step++;`
	`38`	`+ }`
	`39`	`+ return -1;`
	`40`	`+ }`
	`41`	`+}`
	`42`	`+// @lc code=end`
	`43`	`+`

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`50`	`50`	`[221. Maximal Square](https://leetcode.com/problems/maximal-square/) \| [从对应边长找状态转移方程](https://github.com/Yukinichi/leetcode/blob/master/Java/221.maximal-square.java)`
`51`	`51`	`[1143. Longest Common Subsequence](https://leetcode.com/problems/longest-common-subsequence/) \| [LCS问题](https://github.com/Yukinichi/leetcode/blob/master/Java/1143.longest-common-subsequence.java)`
`52`	`52`	`[1312. Minimum Insertion Steps to Make a String Palindrome](https://leetcode.com/problems/minimum-insertion-steps-to-make-a-string-palindrome/) \| [区间dp(小区间->大区间)](https://github.com/Yukinichi/leetcode/blob/master/Java/1312.minimum-insertion-steps-to-make-a-string-palindrome.java)`
	`53`	`+[818. Race Car](https://leetcode.com/problems/race-car/) \| [BFS找最短路径](https://github.com/Yukinichi/leetcode/blob/master/Java/818.race-car.java) \\| [DP](https://github.com/Yukinichi/leetcode/blob/master/Java/818.race-car-dp-solution.java)`
`53`	`54`
`54`	`55`
`55`	`56`	`## Fenwick Tree / Binary Indexed Tree(BIT)`

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