解决思路

回溯

class Solution {
    private   List<String> res;
    private int n;
    String str;
    public List<String> restoreIpAddresses(String s) {
        res = new ArrayList<>();
        n = s.length();
        str = s;
        backtrack(0, "", 4);
        return res;
    }
    //flag为分为4段
    private void backtrack(int i,String temp,int flag){
        if (i == n && flag == 0){
            //去掉最后的.
            res.add(temp.substring(0,temp.length()-1));
            return;
        }
        if (flag < 0) return;
        for (int j = i;j < i + 3 && j < n;j++){
            //对0特别判断
            if (i == j && str.charAt(j) == '0'){
                backtrack(j + 1,temp + str.charAt(j) + ".",flag - 1);
                break;
            }
            //判断是否小于255
            if (Integer.parseInt(str.substring(i,j+1)) <= 255){
                backtrack(j + 1,temp + str.substring(i, j + 1) + ".",flag - 1);
            }
        }
    }
}