402. Remove K Digits ----------M ！！！~~~~~~~！！！！！

Given a non-negative integer num represented as a string, remove k digits from the number so that the new number is the smallest possible.

Note:

• The length of num is less than 10002 and will be ≥ k.
• The given num does not contain any leading zero.

Example 1:

Input: num = "1432219", k = 3
Output: "1219"
Explanation: Remove the three digits 4, 3, and 2 to form the new number 1219 which is the smallest.

Example 2:

Input: num = "10200", k = 1
Output: "200"
Explanation: Remove the leading 1 and the number is 200. Note that the output must not contain leading zeroes.

Example 3:

Input: num = "10", k = 2
Output: "0" 

Explanation: Remove all the digits from the number and it is left with nothing which is 0. 

A:

就是先处理删除掉0 的情况， 然后再每次找到前k+1个digit里最小的，保留之。 

（哎，我一开始还想用快点儿的方法，后来觉得挺麻烦的，就算了， 先用笨方法，然而 结果beat 96.17%.  TNND .  )

class Solution {
public:
    string removeKdigits(string num, int k) {
        // deal with trailing 0 firstly
        int n = num.length();
        for(int i =0;i< min(n, k+1); i++){ //need go further to check num[k],i.e.10200
            if(num[i]=='0'){
                string sub = num.substr(i+1);
                return removeKdigits(sub, k-i);
            }
        }
        return helper(num, k);
    }
private:
    string helper(string num, int k){ // need helper, as we would not deal with heading 0s        
        int n = num.length();
        if(n == 0 || n == k)
            return "0";
        if(k==0)
            return num;
        
        // in first k+1, find the smallest and keep it, remove everything before it.
        char smallestVal = num[0];
        int smallestIndex = 0;
        for(int i =1;i<k+1; i++){ //now n >k, 
            if(num[i] < smallestVal){
                smallestVal = num[i];
                smallestIndex = i;
            }
        }
        string res(1,smallestVal); // use string constructor to convert char to string
        // keep smallest char, remove everything before it
        string subNum = num.substr(smallestIndex+1);
        int toBeDeleted = k - smallestIndex;
        return res + (subNum.length() == toBeDeleted?"" : helper(subNum, toBeDeleted));
    }
};

这里最需要注意的，就是：

要先处理 leading 0 的问题。  然后递归时， 就不处理了。 因此需要helper () 

另外，char to string, 很多时候， 不是可以直接 return "" + 'a'   ;  的， 结果时个数字。 TNND

#include <iostream>

#include <string>

int main(){
	char c = 'A';
	// using string class fill constructor
	std::string s(1, c);
	std::cout << s << '\n';

	return 0;
}