93. Restore IP Addresses

题目：

Given a string containing only digits, restore it by returning all possible valid IP address combinations.

For example:
Given "25525511135",

return ["255.255.11.135", "255.255.111.35"]. (Order does not matter)

链接： http://leetcode.com/problems/restore-ip-addresses/

6/4/2017

6ms, 31%

照着别人改的。注意的问题：

1. restore里面不需要2重循环，直接用i从pos到pos + 3就可以了

2. 第28-30行判断leading 0的方法不错

public class Solution {
    public List<String> restoreIpAddresses(String s) {
        List<String> ret = new ArrayList<>();
        List<String> list = new ArrayList<>();
        restore(s, ret, 0, list);
        return ret;
    }
    private void restore(String s, List<String> ret, int index, List<String> list) {
        if (list.size() == 4) {
            if (index == s.length()) {
                ret.add(new String(String.join(".", list)));
            }
            return;
        }
        for (int i = index; i < s.length() && i <= index + 3; i++) {
            String tmp = s.substring(index, i + 1);
            if (isValid(tmp)) {
                list.add(tmp);
                restore(s, ret, i + 1, list);
                list.remove(list.size() - 1);
            }
        }
    }
    private boolean isValid(String s) {
        if (s == null || s.length() == 0) {
            return false;
        }
        if (s.charAt(0) == '0') {
            return s.length() == 1;
        }
        int num = Integer.parseInt(s);
        if (num < 0 || num >= 256) {
            return false;
        }
        return true;
    }
}

别人的答案：

三重循环

https://discuss.leetcode.com/topic/3919/my-code-in-java

更多讨论

https://discuss.leetcode.com/category/101/restore-ip-addresses