93. Restore IP Addresses

, 
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class Solution {
    public List<String> restoreIpAddresses(String s) {
        List<String> resultList = new ArrayList<>();
        List<Integer> numList = new ArrayList<>(4);
        backtrack(resultList, numList, s, 0);
        return resultList;
    }

    private void backtrack(List<String> resultList, List<Integer> numList, String s, int startIndex) {
        if (numList.size() > 4) {
            return;
        }
        
        if (startIndex == s.length()) {
            if (numList.size() == 4) {
                StringBuilder sb = new StringBuilder();
                for (Integer num : numList) {
                    sb.append(num).append('.');
                }
                sb.deleteCharAt(sb.length() - 1);
                resultList.add(sb.toString());
            }
            return;
        }

        if (s.charAt(startIndex) == '0') {
            numList.add(0);
            backtrack(resultList, numList, s, startIndex + 1);
            numList.remove(numList.size() - 1); // 不要忘记回溯，如 "101023", 如果不进行回溯，那么首个 10 无法被复原，因为当 0 到达时就被作为了新的部分，只有回溯后才能组成首个 10
        } else {
            int num = 0;
            for (int endIndex = startIndex; endIndex < Math.min(startIndex + 3, s.length()); endIndex++) {
                num = num * 10 + (s.charAt(endIndex) - '0');
                if (num <= 255) {
                    numList.add(num);
                    backtrack(resultList, numList, s, endIndex + 1);
                    numList.remove(numList.size() - 1); // 不要忘记回溯
                }
            }
        }
    }
}

注意对 0 的特殊处理及 IP 地址必须由 4 个数字组成。

References

93. Restore IP Addresses
剑指 Offer II 087. 复原 IP