DataStructure_Algorithm_HandBook_PreForLeetCode

93. Restore IP Addresses

A valid IP address consists of exactly four integers separated by single dots. Each integer is between 0 and 255 (inclusive) and cannot have leading zeros.

• For example, "0.1.2.201" and "192.168.1.1" are valid IP addresses, but "0.011.255.245", "192.168.1.312" and "192.168@1.1" are invalid IP addresses.

Given a string s containing only digits, return all possible valid IP addresses that can be formed by inserting dots into s. You are not allowed to reorder or remove any digits in s. You may return the valid IP addresses in any order.

Example 1:

Input: s = "25525511135"
Output: ["255.255.11.135","255.255.111.35"]

Example 2:

Input: s = "0000"
Output: ["0.0.0.0"]

Example 3:

Input: s = "101023"
Output: ["1.0.10.23","1.0.102.3","10.1.0.23","10.10.2.3","101.0.2.3"]

Constraints:

• 1 <= s.length <= 20
• s consists of digits only.

Solution

这道题目结合了具体的场景，会有一点难度。 每个组合可以是单独的 0，但是不能是前置 0 的多位数

我们可以想象有一个数组，然后有 4 个空位，我们要把 s 中的元素填充到这个数组里面去，比如第一个元素我们可以填写 255，第二个也可以填 255.

模拟这个过程。

我们可以来一个 segment 数组，然后 segmentid 代表数组的位置，然后 index 代表整个字符串的下标。每次我们都是从（index，index+3）的位置来选取。

只有当 segid == 5 以及 index == length 的时候才满足条件的 ip

var restoreIpAddresses = function(s) {
    let result = []; 
    dfs(0, [], s, 0,result);
    return result;
};

function isValidIPv4(str) {
    if (str.length > 1 && str.charAt(0) === '0') return false; // 排除前导零的情况，除非是“0”
    const num = parseInt(str);
    return num <= 255; // 数字必须小于或等于255
}

function dfs(segid, segments, s, index,result) {
    if (segid === 4 && index === s.length) {
        result.push(segments.join('.')); // 当且仅当用完所有段且索引覆盖了整个字符串时，加入结果
        return;
    }
    if (segid === 4 || index === s.length) return; // 提前终止不合理的递归路径

    for (let len = 1; len <= 3 && index + len <= s.length; len++) { // 检查从1到3个字符的可能性
        const sub = s.substring(index, index + len);
        if (isValidIPv4(sub)) {
            segments.push(sub);
            dfs(segid + 1, segments, s, index + len,result);
            segments.pop(); // 回溯，移除最后添加的段
        }
    }
}

var restoreIpAddresses = function(s) {
    let result = [];
    dfs(0,[],0,s,result);
    return result; 
}
function isVaild(str){
    if(str.length > 1 && str.charAt(0) ==="0") return false;
    const num = parseInt(str);
    return num <= 255; // 数字必须小于或等于255
}
function dfs(segid, segments, index , s, result){
    if(segid === 4 && index == s.length ){
        result.push(segments.join("."));
        return;
    }
    for (let i = 1 ;i <= 3 && i+index <= s.length; i++ ){
        let sub = s.substring(index,index+i);
        if (isVaild(sub)){
              segments.push(sub);  
              dfs(segid+1,segments,index+i,s,result);
              segments.pop();
        }
    }
}