LeetCode 1064. Fixed Point

Problem

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?

	class Solution {
	public:
	int fixedPoint(vector<int>& arr) {
	int l = 0;
	int r = arr.size() - 1;
	while(l < r){
	int mid = (l + r) / 2;
	if(arr[mid]>=mid)
	r = mid;
	else
	l = mid + 1;
	}
	if(arr[l] == l) return l;
	return -1;
	}
	};

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