First Element Equals its Index

Let's check another problem with a sorted input array. Difficulty Level: Easy

Problem statement

Suppose we are given a sorted array of distinct integers, nums, which are sorted in ascending order. We will return the smallest index i, which satisfies nums[i] == i. If no such i exists, we will return -1.


  • 1nums.length1031 \leq nums.length \leq 10^3
  • 103nums[i]103-10^3 \leq nums[i] \leq 10^3

Example 1: General case

Input: [-10,-5,0,3,7]    
Output: 3    

Explanation: For the given array, nums[0] = -10, nums[1] = -5, nums[2] = 0, nums[3] = 3, the output is 3.

Example 2: The answer is at the beginning of the array

Input: [0,2,5,8,17]    
Output: 0    

Explanation: nums[0] = 0. Thus the output is 0.

Example 3: The answer is missing

Input: [-10,-5,3,4,7,9]    
Output: -1    

Explanation: There is no such i that nums[i] = i. Thus the output is -1.

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