852. Peak Index in a Mountain Array

Let's call an array arr a mountain if the following properties hold:

  • arr.length >= 3
  • There exists some i with 0 < i < arr.length - 1 such that:
    • arr[0] < arr[1] < ... arr[i-1] < arr[i]
    • arr[i] > arr[i+1] > ... > arr[arr.length - 1]

Given an integer array arr that is guaranteed to be a mountain, return any i such that arr[0] < arr[1] < ... arr[i - 1] < arr[i] > arr[i + 1] > ... > arr[arr.length - 1].

 

Example 1:

Input: arr = [0,1,0]
Output: 1

Example 2:

Input: arr = [0,2,1,0]
Output: 1

Example 3:

Input: arr = [0,10,5,2]
Output: 1

Example 4:

Input: arr = [3,4,5,1]
Output: 2

Example 5:

Input: arr = [24,69,100,99,79,78,67,36,26,19]
Output: 2

 

Constraints:

  • 3 <= arr.length <= 104
  • 0 <= arr[i] <= 106
  • arr is guaranteed to be a mountain array.

 

Follow up: Finding the O(n) is straightforward, could you find an O(log(n)) solution?

Rust Solution

struct Solution;

impl Solution {
    fn peak_index_in_mountain_array(a: Vec<i32>) -> i32 {
        let mut l: usize = 0;
        let mut r: usize = a.len() - 1;
        while l < r {
            let m = (l + r) / 2;
            if a[m] < a[m + 1] {
                l = m + 1;
            } else {
                r = m;
            }
        }
        l as i32
    }
}

#[test]
fn test() {
    let a = vec![0, 1, 0];
    assert_eq!(Solution::peak_index_in_mountain_array(a), 1);
    let a = vec![0, 2, 1, 0];
    assert_eq!(Solution::peak_index_in_mountain_array(a), 1);
}

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