improving java solution by using stringBuilder

problems/0093.复原IP地址.md

@@ -314,6 +314,47 @@ class Solution {
         return true;
     }
 }
+//方法一：但使用stringBuilder，故优化时间、空间复杂度，因为向字符串插入字符时无需复制整个字符串，从而减少了操作的时间复杂度，也不用开新空间存subString，从而减少了空间复杂度。
+class Solution {
+    List<String> result = new ArrayList<>();
+    public List<String> restoreIpAddresses(String s) {
+        StringBuilder sb = new StringBuilder(s);
+        backTracking(sb, 0, 0);
+        return result;
+    }
+    private void backTracking(StringBuilder s, int startIndex, int dotCount){
+        if(dotCount == 3){
+            if(isValid(s, startIndex, s.length() - 1)){
+                result.add(s.toString());
+            }
+            return;
+        }
+        for(int i = startIndex; i < s.length(); i++){
+            if(isValid(s, startIndex, i)){
+                s.insert(i + 1, '.');
+                backTracking(s, i + 2, dotCount + 1);
+                s.deleteCharAt(i + 1);
+            }else{
+                break;
+            }
+        }
+    }
+    //[start, end]
+    private boolean isValid(StringBuilder s, int start, int end){
+        if(start > end)
+            return false;
+        if(s.charAt(start) == '0' && start != end)
+            return false;
+        int num = 0;
+        for(int i = start; i <= end; i++){
+            int digit = s.charAt(i) - '0';
+            num = num * 10 + digit;
+            if(num > 255)
+                return false;
+        }
+        return true;
+    }
+}
 
 //方法二：比上面的方法时间复杂度低，更好地剪枝，优化时间复杂度
 class Solution {
@@ -358,6 +399,7 @@ class Solution {
 }
 ```
 
+
 ## python
 
 python2: