[LeetCode] 962. Maximum Width Ramp

A ramp in an integer array nums is a pair (i, j) for which i < j and nums[i] <= nums[j]. The width of such a ramp is j - i.

Given an integer array nums, return the maximum width of a ramp in nums. If there is no ramp in nums, return 0.

Example 1:
Input: nums = [6,0,8,2,1,5]
Output: 4
Explanation: The maximum width ramp is achieved at (i, j) = (1, 5): nums[1] = 0 and nums[5] = 5.

Example 2:
Input: nums = [9,8,1,0,1,9,4,0,4,1]
Output: 7
Explanation: The maximum width ramp is achieved at (i, j) = (2, 9): nums[2] = 1 and nums[9] = 1.

Constraints:
2 <= nums.length <= 5 * 104
0 <= nums[i] <= 5 * 104

最大寬度坡。

給定一個整數陣列 A，坡是元組 (i, j)，其中 i < j 且 A[i] <= A[j]。這樣的坡的寬度為 j - i。

找出 A 中的坡的最大寬度，如果不存在，返回 0 。

思路

這道題的思路是單調棧，不過這道題跟一般的單調棧題目有些不同。這道題的單調棧找的是兩個距離最遠的下標。

複雜度

時間O(n)
空間O(n)

程式碼

Java實現

class Solution {
    public int maxWidthRamp(int[] nums) {
        Deque<Integer> stack = new ArrayDeque<>();
        int res = 0;
        int n = nums.length;
        for (int i = 0; i < n; i++) {
            if (stack.isEmpty() || nums[i] < nums[stack.peekLast()]) {
                stack.offerLast(i);
            }
        }

        for (int i = n - 1; i > 0; i--) {
            while (!stack.isEmpty() && nums[i] >= nums[stack.peekLast()]) {
                res = Math.max(res, i - stack.pollLast());
            }
        }
        return res;
    }
}