Peak index in a mountain array

Array Binary Search

66.9% acceptance

Feb 22, 2026

8459

1951

You are given an integer mountain array arr of length n where the values increase to a peak element and then decrease. Return the index of the peak element. Your task is to solve it in O(log(n)) time complexity.

Solution

Rust

Time O(n log n)

Space O(1)

LeetCode

solution.rs

/*
 * You are given an integer mountain array arr of length n where the values increase to a peak element and then decrease.
 * Return the index of the peak element.
 * Your task is to solve it in O(log(n)) time complexity.
 * 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
 * Constraints:
 * 3 <= arr.length <= 105
 * 0 <= arr[i] <= 106
 * arr is guaranteed to be a mountain array.
 */

impl Solution {
  pub fn peak_index_in_mountain_array(arr: Vec<i32>) -> i32 {
    let mut lo = 0usize;
    let mut hi = arr.len() - 1;
    while lo < hi {
      let mid = (lo + hi) / 2;
      if arr[mid] < arr[mid + 1] { lo = mid + 1; }
      else { hi = mid; }
    }
    lo as i32
  }
}