402. Remove K Digits

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.

Solution:

class Solution {

    /**
     * @param String $num
     * @param Integer $k
     * @return String
     */
    function removeKdigits($num, $k) {
        if (strlen($num) == $k) {
            return '0';
        }

        $ans = [];
        $stack = [];

        for ($i = 0; $i < strlen($num); $i++) {
            $digit = $num[$i];
            while ($k > 0 && !empty($stack) && $stack[count($stack) - 1] > $digit) {
                array_pop($stack);
                $k -= 1;
            }
            array_push($stack, $digit);
        }

        for ($j = 0; $j < $k; $j++) {
            array_pop($stack);
        }

        foreach ($stack as $c) {
            if ($c == '0' && empty($ans)) {
                continue;
            }
            array_push($ans, $c);
        }

        return implode($ans) ? implode($ans) : '0';
    }
}

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