README.md

402. Remove K Digits

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.

Solutions (Rust)

1. Solution

impl Solution {
    pub fn remove_kdigits(num: String, k: i32) -> String {
        let keep = num.len() - k as usize;
        let mut stack = vec![];

        for (i, digit) in num.bytes().enumerate() {
            while *stack.last().unwrap_or(&b'0') > digit && num.len() - i + stack.len() > keep {
                stack.pop();
            }

            if stack.len() < keep {
                stack.push(digit);
            }
        }

        if stack.iter().all(|&digit| digit == b'0') {
            return "0".to_string();
        }

        String::from_utf8(stack)
            .unwrap()
            .trim_start_matches('0')
            .to_string()
    }
}