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## Problem

Given string num representing a non-negative integer num, and an integer k, return the smallest possible integer after removing k digits from num.

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.

## Code

class Solution {
public String removeKdigits(String num, int k) {
if(num.length() <= k) return "0";

Stack<Integer> stack = new Stack<>();

for(int i = 0; i < num.length(); i++){
int temp = num.charAt(i) - '0';
while(!stack.isEmpty() && k > 0 && stack.peek() > temp){
stack.pop();
k--;
}

stack.push(temp);
}

// 排除剩余的数字
while(k > 0){
stack.pop();
k--;
}

StringBuilder sb = new StringBuilder();
while(!stack.isEmpty()){
sb.append(stack.pop());
}

sb.reverse();

while(sb.length() != 0 && sb.charAt(0) == '0'){
sb.deleteCharAt(0);
}

return sb.length() == 0 ? "0" : sb.toString();
}
}