LeetCode-in-Java

2289. Steps to Make Array Non-decreasing

Medium

You are given a 0-indexed integer array nums. In one step, remove all elements nums[i] where nums[i - 1] > nums[i] for all 0 < i < nums.length.

Return the number of steps performed until nums becomes a non-decreasing array.

Example 1:

Input: nums = [5,3,4,4,7,3,6,11,8,5,11]

Output: 3

Explanation: The following are the steps performed:

• Step 1: [5,3,4,4,7,3,6,11,8,5,11] becomes [5,4,4,7,6,11,11]

• Step 2: [5,4,4,7,6,11,11] becomes [5,4,7,11,11]

• Step 3: [5,4,7,11,11] becomes [5,7,11,11]

[5,7,11,11] is a non-decreasing array. Therefore, we return 3.

Example 2:

Input: nums = [4,5,7,7,13]

Output: 0

Explanation: nums is already a non-decreasing array. Therefore, we return 0.

Constraints:

• 1 <= nums.length <= 105
• 1 <= nums[i] <= 109

Solution

public class Solution {
    public int totalSteps(int[] nums) {
        int max = 0;
        int[] pos = new int[nums.length + 1];
        int[] steps = new int[nums.length + 1];
        int top = -1;
        for (int i = 0; i <= nums.length; i++) {
            int val = i == nums.length ? Integer.MAX_VALUE : nums[i];
            while (top >= 0 && nums[pos[top]] <= val) {
                if (top == 0) {
                    max = Math.max(max, steps[pos[top--]]);
                } else {
                    steps[pos[--top]] = Math.max(steps[pos[top]] + 1, steps[pos[top + 1]]);
                }
            }
            pos[++top] = i;
        }
        return max;
    }
}

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