402. Remove K Digits

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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();
    }
}