g0401_0500.s0402_remove_k_digits.Solution Maven / Gradle / Ivy

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Java-based LeetCode algorithm problem solutions, regularly updated

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package g0401_0500.s0402_remove_k_digits;

// #Medium #String #Greedy #Stack #Monotonic_Stack
// #2022_07_15_Time_4_ms_(99.66%)_Space_42.3_MB_(99.36%)

/**
 * 402 - Remove K Digits\.
 *
 * Medium
 *
 * 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. 
 *
 * **Constraints:**
 *
 * *   1 <= k <= num.length <= 10⁵
 * *   `num` consists of only digits.
 * *   `num` does not have any leading zeros except for the zero itself.
**/
public class Solution {
    public String removeKdigits(String num, int k) {
        char[] list = new char[num.length()];
        int len = num.length() - k;
        int top = 0;
        for (int i = 0; i < num.length(); i++) {
            while (top > 0 && k > 0 && num.charAt(i) < list[top - 1]) {
                top--;
                k--;
            }
            list[top++] = num.charAt(i);
        }
        int number = 0;
        while (number < len && list[number] == '0') {
            number++;
        }
        return number == len ? "0" : new String(list, number, len - number);
    }
}