更新0093.复原IP地址python3版本，添加剪枝操作

problems/0093.复原IP地址.md

@@ -320,14 +320,15 @@ class Solution:
             if p[0] == '0': return False
             if int(p) > 0 and int(p) <256: return True
             return False
-        
+
         def backtrack(s, startIndex):
             if len(s) > 12: return  # 字符串长度最大为12
             if len(path) == 4 and startIndex == len(s): # 确保切割完，且切割后的长度为4
                 res.append(".".join(path[:])) # 字符拼接
-                return 
+                return
 
             for i in range(startIndex, len(s)):
+                if len(s) - startIndex > 3*(4 - len(path)): continue # 剪枝，剩下的字符串大于允许的最大长度则跳过
                 p = s[startIndex:i+1] # 分割字符
                 if isValid(p): # 判断字符是否有效
                     path.append(p)