962. Maximum Width Ramp

Given an array A of integers, a ramp is a tuple (i, j) for which i < j and A[i] <= A[j].  The width of such a ramp is j - i.

Find the maximum width of a ramp in A.  If one doesn't exist, return 0.

 

Example 1:

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

Example 2:

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

 

Note:

  1. 2 <= A.length <= 50000
  2. 0 <= A[i] <= 50000
 

Rust Solution

struct Solution;

impl Solution {
    fn max_width_ramp(a: Vec<i32>) -> i32 {
        let mut stack: Vec<usize> = vec![];
        let n = a.len();
        for i in 0..n {
            if let Some(&j) = stack.last() {
                if a[i] < a[j] {
                    stack.push(i);
                }
            } else {
                stack.push(i);
            }
        }
        let mut res = 0;
        for i in (0..n).rev() {
            while let Some(&j) = stack.last() {
                if a[j] <= a[i] {
                    res = res.max(i - j);
                    stack.pop();
                } else {
                    break;
                }
            }
        }
        res as i32
    }
}

#[test]
fn test() {
    let a = vec![6, 0, 8, 2, 1, 5];
    let res = 4;
    assert_eq!(Solution::max_width_ramp(a), res);
    let a = vec![9, 8, 1, 0, 1, 9, 4, 0, 4, 1];
    let res = 7;
    assert_eq!(Solution::max_width_ramp(a), res);
}

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