(回溯)数字字符串转化成IP地址

/**
  * 
  * @param s string字符串 
  * @return string字符串一维数组
  */
function restoreIpAddresses( s ) {
    // write code here
    let res = [];
    let tmp = [];
    if(s.length < 4) {
        return res;
    }
    dfs(s, res, tmp);
    return res;
}
function dfs(s, res, tmp) {
    if (s.length === 0 && tmp.length === 4) res.push(convert(tmp));
    if (tmp.length === 3 && (s > 255))  return;
    for (let i = 1; i <= s.length; ++i) {
        if (i > 1 && s[0] == '0') break;
        let val = s.substr(0, i);
        if (val > 255) break;
        if(tmp.length  >= 4) break;
        tmp.push(s.substr(0, i));
        dfs(s.substr(i), res, tmp);
        tmp.pop();
    }
}
function convert(tmp) {
    return `${tmp[0]}.${tmp[1]}.${tmp[2]}.${tmp[3]}`;
}
module.exports = {
    restoreIpAddresses : restoreIpAddresses
};