RemoveKDigitsToGetSmallestNumber.java

package Leetcode;

import java.util.ArrayDeque;
import java.util.Deque;

/**
 * @author kalpak
 *
 * Given a non-negative integer num represented as a string,
 * remove k digits from the number so that the new number is the smallest possible.
 *
 * Note:
 * The length of num is less than 10002 and will be ≥ k.
 * The given num does not contain any leading zero.
 *
 * 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.
 *
 */

public class RemoveKDigitsToGetSmallestNumber {
    public static String removeKdigits(String num, int k) {
        Deque<Character> stack = new ArrayDeque<>();

        for(char digit : num.toCharArray()) {
            while(stack.size() > 0 && k > 0 && stack.peekLast() > digit) {
                stack.removeLast();
                k -= 1;
            }

            stack.addLast(digit);
        }

        while(k-- > 0)
            stack.removeLast();

        StringBuilder result = new StringBuilder();
        boolean leadingZero = true;

        for(char digit : stack) {
            if(leadingZero && digit == '0')
                continue;

            leadingZero = false;
            result.append(digit);
        }

        return (result.length() == 0) ? "0" : result.toString();
    }

    public static void main(String[] args) {
        System.out.println("The smallest possible number after K removals is : " + removeKdigits("1432219", 3));
    }
}