1064. Fixed Point

Given an array of distinct integers arr, where arr is sorted in ascending order, return the smallest index i that satisfies arr[i] == i. If there is no such index, return -1.

 

Example 1:

Input: arr = [-10,-5,0,3,7]
Output: 3
Explanation: For the given array, arr[0] = -10, arr[1] = -5, arr[2] = 0, arr[3] = 3, thus the output is 3.

Example 2:

Input: arr = [0,2,5,8,17]
Output: 0
Explanation: arr[0] = 0, thus the output is 0.

Example 3:

Input: arr = [-10,-5,3,4,7,9]
Output: -1
Explanation: There is no such i that arr[i] == i, thus the output is -1.

 

Constraints:

  • 1 <= arr.length < 104
  • -109 <= arr[i] <= 109

 

Follow up: The O(n) solution is very straightforward. Can we do better?

Rust Solution

struct Solution;

impl Solution {
    fn fixed_point(a: Vec<i32>) -> i32 {
        for i in 0..a.len() {
            if i as i32 == a[i] {
                return i as i32;
            }
        }
        -1
    }
}

#[test]
fn test() {
    assert_eq!(Solution::fixed_point(vec![-10, -5, 0, 3, 7]), 3);
    assert_eq!(Solution::fixed_point(vec![0, 2, 5, 8, 17]), 0);
    assert_eq!(Solution::fixed_point(vec![-10, -5, 3, 4, 7, 9]), -1);
}

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