Merge pull request #2638 from robotsouper/master

更新了Leetcode 131 java部分的写法

problems/0131.分割回文串.md

@@ -310,39 +310,35 @@ public:
 ### Java 
 ```Java
 class Solution {
-    List<List<String>> lists = new ArrayList<>();
-    Deque<String> deque = new LinkedList<>();
-
+    //保持前几题一贯的格式， initialization
+    List<List<String>> res = new ArrayList<>();
+    List<String> cur = new ArrayList<>();
     public List<List<String>> partition(String s) {
-        backTracking(s, 0);
-        return lists;
+        backtracking(s, 0, new StringBuilder());
+        return res;
     }
-
-    private void backTracking(String s, int startIndex) {
-        //如果起始位置大于s的大小，说明找到了一组分割方案
-        if (startIndex >= s.length()) {
-            lists.add(new ArrayList(deque));
+    private void backtracking(String s, int start, StringBuilder sb){
+        //因为是起始位置一个一个加的，所以结束时start一定等于s.length,因为进入backtracking时一定末尾也是回文，所以cur是满足条件的
+        if (start == s.length()){
+            //注意创建一个新的copy
+            res.add(new ArrayList<>(cur));
             return;
         }
-        for (int i = startIndex; i < s.length(); i++) {
-            //如果是回文子串，则记录
-            if (isPalindrome(s, startIndex, i)) {
-                String str = s.substring(startIndex, i + 1);
-                deque.addLast(str);
-            } else {
-                continue;
+        //像前两题一样从前往后搜索，如果发现回文，进入backtracking,起始位置后移一位，循环结束照例移除cur的末位
+        for (int i = start; i < s.length(); i++){
+            sb.append(s.charAt(i));
+            if (check(sb)){
+                cur.add(sb.toString());
+                backtracking(s, i + 1, new StringBuilder());
+                cur.remove(cur.size() -1 );
             }
-            //起始位置后移，保证不重复
-            backTracking(s, i + 1);
-            deque.removeLast();
         }
     }
-    //判断是否是回文串
-    private boolean isPalindrome(String s, int startIndex, int end) {
-        for (int i = startIndex, j = end; i < j; i++, j--) {
-            if (s.charAt(i) != s.charAt(j)) {
-                return false;
-            }
+
+    //helper method, 检查是否是回文
+    private boolean check(StringBuilder sb){
+        for (int i = 0; i < sb.length()/ 2; i++){
+            if (sb.charAt(i) != sb.charAt(sb.length() - 1 - i)){return false;}
         }
         return true;
     }