93. Restore IP Addresses

This MiHoYo coding test question is very similar to LeetCode 46 (Permutations), and both rely on the backtracking approach.

As shown in the code, during the first traversal, we may get something like ['2', '5', '5', '2'] as our initial parts, but at this point we haven't traversed the entire string yet.

When we enter the next level of DFS, the pointer moves forward by +length, effectively pointing to the far right of the current segment. If the pointer has reached the end of the string, it means we've visited all characters — we've found one valid answer and can add it to the result list.

One important note: parts + [part] is pass-by-value, not by reference like in LeetCode 46. This means we don't need to manually undo changes (no need to backtrack with pop()), because each recursive call creates a new list.

Code

from typing import List

class Solution:
    def restoreIpAddresses(self, s: str) -> List[str]:
        res = []

        def backtrack(start: int, parts: List[str]):
            # 终止条件：正好4段且用完所有字符
            # Stop condition: exactly 4 segments and all characters used up
            if len(parts) == 4:
                if start == len(s):
                    res.append(".".join(parts))
                return

            for length in range(1, 4):  # 每段长度1~3 Each segment length 1~3
                if start + length > len(s):
                    break
                part = s[start:start+length]

                # 前导0非法，但0本身合法
                # Leading 0 is illegal, but 0 itself is legal
                if len(part) > 1 and part[0] == '0':
                    continue

                if int(part) <= 255:
                    backtrack(start + length, parts + [part])  # 注意用 + 避免污染 We need to use + to avoid pollution

        backtrack(0, [])
        return res

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longsizhuo贡献 4 次 · 最近 2026/05/06

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93. Restore IP Addresses

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