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website/content/ChapterFour/0093.Restore-IP-Addresses.md

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+# [93. Restore IP Addresses](https://leetcode.com/problems/restore-ip-addresses/)
+
+
+## 题目
+
+Given a string containing only digits, restore it by returning all possible valid IP address combinations.
+
+**Example**:
+
+    Input: "25525511135"
+    Output: ["255.255.11.135", "255.255.111.35"]
+
+## 题目大意
+
+给定一个只包含数字的字符串，复原它并返回所有可能的 IP 地址格式。
+
+## 解题思路
+
+- DFS 深搜
+- 需要注意的点是 IP 的规则，以 0 开头的数字和超过 255 的数字都为非法的。
+
+
+
+## 代码
+
+```go
+
+package leetcode
+
+import (
+	"strconv"
+)
+
+func restoreIPAddresses(s string) []string {
+	if s == "" {
+		return []string{}
+	}
+	res, ip := []string{}, []int{}
+	dfs(s, 0, ip, &res)
+	return res
+}
+
+func dfs(s string, index int, ip []int, res *[]string) {
+	if index == len(s) {
+		if len(ip) == 4 {
+			*res = append(*res, getString(ip))
+		}
+		return
+	}
+	if index == 0 {
+		num, _ := strconv.Atoi(string(s[0]))
+		ip = append(ip, num)
+		dfs(s, index+1, ip, res)
+	} else {
+		num, _ := strconv.Atoi(string(s[index]))
+		next := ip[len(ip)-1]*10 + num
+		if next <= 255 && ip[len(ip)-1] != 0 {
+			ip[len(ip)-1] = next
+			dfs(s, index+1, ip, res)
+			ip[len(ip)-1] /= 10
+		}
+		if len(ip) < 4 {
+			ip = append(ip, num)
+			dfs(s, index+1, ip, res)
+			ip = ip[:len(ip)-1]
+		}
+	}
+}
+
+func getString(ip []int) string {
+	res := strconv.Itoa(ip[0])
+	for i := 1; i < len(ip); i++ {
+		res += "." + strconv.Itoa(ip[i])
+	}
+	return res
+}
+
+```