LeetCode 93. Restore IP Addresses（恢复IP地址）

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原题网址：https://leetcode.com/problems/restore-ip-addresses/

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)

方法：深度优先搜索。

public class Solution {    /*"0000""00000""25525511135""9245587303""0279245587303""010010"    */    private void find(String s, int[] pos, int step, List<String> addresses) {        if (step == pos.length-1) {            if (pos[step]-pos[step-1]>3) return;            if (pos[step]-pos[step-1]>1 && s.charAt(pos[step-1]) == '0') return;            int a = Integer.parseInt(s.substring(pos[step-1]), 10);            if (a>255) return;            StringBuilder sb = new StringBuilder();            for(int i=0; i<pos.length-1; i++) {                if (i>0) sb.append(".");                sb.append(s.substring(pos[i], pos[i+1]));            }            addresses.add(sb.toString());            return;        }        // Set<Integer> used = new HashSet<>();        for(int i=pos[step-1]+1; i+(3-step)<pos[pos.length-1]; i++) {            int a = Integer.parseInt(s.substring(pos[step-1], i), 10);            if (a>255) break;            // if (used.contains(a)) continue;            // used.add(a);            pos[step] = i;            find(s, pos, step+1, addresses);            if (a==0) break;        }    }    public List<String> restoreIpAddresses(String s) {        List<String> addresses = new ArrayList<>();        int[] pos = new int[5];        pos[0] = 0;        pos[4] = s.length();        find(s, pos, 1, addresses);        return addresses;    }}

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