【回溯】【回文字符串】131.分割回文串

题目

法1：DFS+双指针

必须掌握基础方法！
注意：使用ArrayList删除尾元素比LinkedList要快很多！！！

class Solution {public List<List<String>> partition(String s) {List<List<String>> res = new ArrayList<>();if (s.length() == 0) {return res;}List<String> tmp = new ArrayList<>();dfs(s, 0, tmp, res);return res;}public void dfs(String s, int startIndex, List<String> tmp, List<List<String>> res) {if (startIndex == s.length()) {res.add(new ArrayList<>(tmp));return;}for (int i = startIndex; i < s.length(); ++i) {if (!valid(s, startIndex, i)) {continue;}tmp.add(s.substring(startIndex, i + 1));dfs(s, i + 1, tmp, res);tmp.remove(tmp.size() - 1);}}public boolean valid(String s, int startIndex, int endIndex) {while (startIndex <= endIndex) {char left = s.charAt(startIndex);char right = s.charAt(endIndex);if (left != right) {return false;}++startIndex;--endIndex;}return true;}
}

法2：DFS+回文串判断预处理

主要优化回文串判断部分

class Solution {public List<List<String>> partition(String s) {List<List<String>> res = new ArrayList<>();int n = s.length();if (n == 0) {return res;}boolean[][] valid = new boolean[n][n];List<String> tmp = new ArrayList<>();for (int i = 0; i < n; ++i) {Arrays.fill(valid[i], true);}for (int i = n - 1; i >= 0; --i) {for (int j = i + 1; j < n; ++j) {valid[i][j] = (s.charAt(i) == s.charAt(j)) && valid[i + 1][j - 1];}}dfs(s, 0, tmp, res, valid);return res;}public void dfs(String s, int startIndex, List<String> tmp, List<List<String>> res, boolean[][] valid) {if (startIndex == s.length()) {res.add(new ArrayList<>(tmp));return;}for (int i = startIndex; i < s.length(); ++i) {if (valid[startIndex][i] == false) {continue;}tmp.add(s.substring(startIndex, i + 1));dfs(s, i + 1, tmp, res, valid);tmp.remove(tmp.size() - 1);}}
}