题解 | #数字字符串转化成IP地址#

#
# 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
#
# 
# @param s string字符串 
# @return string字符串一维数组
#
class Solution:
    def restoreIpAddresses(self , s: str) -> List[str]:
        # write code here
        res = []
        ip = []
        n = len(s)

        def backtrack(s):
            # IP接收数组满四个并且s为空，检查并存储结果
            if len(ip) == 4 and s == '':
                ips = '.'.join(ip)
                if ips not in res:
                    res.append(ips)
                return
            # 空字符s，或者s未用完但ip够了，不合法，返回
            if s == '' or len(ip) == 4 and s != '':
                return
			
            # 回溯截取[1,2,3]长度的字符
            for i in range(1, 4):
                sub = s[:i]
                # 排除不合法IP字段
                if int(sub) > 255 or len(sub) > 1 and sub[0] == '0':
                    continue
                ip.append(sub)
                backtrack(s[i:])
                ip.pop()
        
        backtrack(s)
        return res