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# 題目敘述

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 0.5)

# 解題思路

# Solution

import java.util.Stack;
class Solution {
    public String removeKdigits(String num, int k) {
        if (num.length() == k) {
            return "0";
        }
        
        Stack<Character> stack = new Stack<>();
        
        for (char c : num.toCharArray()) {
            while (k > 0 && !stack.isEmpty() && stack.peek() > c) {
                stack.pop();
                k--;
            }
            stack.push(c);
        }
        
        while (k > 0) {
            stack.pop();
            k--;
        }
        
        StringBuilder sb = new StringBuilder();
        while (!stack.isEmpty()) {
            sb.append(stack.pop());
        }
        
        sb.reverse();
        
        while (sb.length() > 1 && sb.charAt(0) == '0') {
            sb.deleteCharAt(0);
        }
        
        return sb.toString();
    }
}




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