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Find the Most Competitive Subsequence

Updated: Jan 21, 2021

Given an integer array nums and a positive integer k, return the most competitive subsequence of nums of size k. An array's subsequence is a resulting sequence obtained by erasing some (possibly zero) elements from the array.

We define that a subsequence a is more competitive than a subsequence b (of the same length) if in the first position where a and b differ, subsequence a has a number less than the corresponding number in b. For example, [1,3,4] is more competitive than [1,3,5] because the first position they differ is at the final number, and 4 is less than 5.

Example 1:

Input: nums = [3,5,2,6], k = 2 
Output: [2,6] 
Explanation: Among the set of every possible subsequence: {[3,5], [3,2], [3,6], [5,2], [5,6], [2,6]}, [2,6] is the most competitive. 

Example 2:

Input: nums = [2,4,3,3,5,4,9,6], k = 4 Output: [2,3,3,4]  

Constraints:

  • 1 <= nums.length <= 105

  • 0 <= nums[i] <= 109

  • 1 <= k <= nums.length

Solution:

class Solution {
    public int[] mostCompetitive(int[] nums, int k) {
        Stack<Integer> stack = new Stack<Integer>();
        int n=nums.length;
        for(int i=0;i<n;i++)
        {
          while(!stack.isEmpty() && nums[i]<stack.peek() && n-i+stack.size()>k)
            {
                stack.pop();
            }
            if(stack.size()<k)
                stack.push(nums[i]);
        }
        int[] ans = new int[k];
        int index = k-1;
        while(!stack.isEmpty())
        {
            ans[index--] = stack.pop();
        }
        return ans;
    }
}


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