Remove K Digits Solutions in C++

// Source : https://leetcode.com/problems/remove-k-digits/
// Author : Hao Chen
// Date   : 2016-11-11



class Solution {
public:
    string removeKdigits_pick(string& num, int k) {
        
        int len = num.size();
        string result;
        
        int idx = 0;
        for (int i=0; i < len - k; i++) {
            int min_idx = idx;
            for (int j=min_idx; j<=i+k; j++) {
                if (num[min_idx] > num[j])  min_idx = j;
            }
            
            //don't put zero at the beginning
            if ( !(result.empty() && num[min_idx]=='0') ){
                result.push_back(num[min_idx]);
            }
            
            //select the number started from next one, to make the order correctness.
            idx = min_idx + 1;
        }
        
        if (result.empty()) result = "0";
        return result;
    }
    
    string removeKdigits_remove(string& num, int k) {
        if ( num.size() <= k ) return "0";
        int left_len = num.size() - k;
        int idx = 0;
        for (int i=0; i<k ;i++){
            int len = num.size();
            for (int j=0; j<num.size()-1; j++) {
                //if the current is bigger than next one, then revmoe the current one.
                //In other word, we always pick the smaller one number.
                if ( num[j] > num[j+1] ) {
                    num.erase(j, 1);
                    idx = j;
                    break;
                }
            }
        }
        
        //remove all of ZEROs at the beginning.
        for (int i=0; i<= num.size(); i++) {
            if (num[i] != '0' || i == num.size()) {
                num.erase(0, i);
                break;
            }
        }
        
        // if the digits in the array are sorted, 
        // then, we need remove the digits at the ends.
        if (num.size() > left_len ) {
            num.erase(num.begin() + left_len, num.end());
        }
        
        if (num.empty()) num = "0";
        return num;
    }
    
    string removeKdigits(string num, int k) {
        srand(time(0));
        if (rand() % 2 ) {
            return removeKdigits_pick(num, k);
        } else {
            return removeKdigits_remove(num, k);
        }
    }
};

/// Source : https://leetcode.com/problems/remove-k-digits/
/// Author : liuyubobobo
/// Time   : 2020-05-21

#include <iostream>
#include <stack>

using namespace std;


/// Monotone Stack
/// Time Complexity: O(n)
/// Space Complexity: O(n)
class Solution {
public:
    string removeKdigits(string num, int k) {

        string res = "";
        for(int i = 0; i < num.size(); i ++){
            while(res.size() && res.back() > num[i] && k){
                res.pop_back();
                k --;
            }
            res += num[i];
        }

        while(k) res.pop_back(), k --;

        int start = 0;
        while(start < res.size() && res[start] == '0') start ++;
        res = res.substr(start);

        return res.size() ? res : "0";
    }
};


int main() {

    cout << Solution().removeKdigits("10", 2) << endl;
    // 0

    cout << Solution().removeKdigits("112", 1) << endl;
    // 11

    return 0;
}