852. Peak Index in a Mountain Array

852. Peak Index in a Mountain Array (Easy)

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

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

Given an array that is definitely a mountain, return any i such that A[0] < A[1] < ... A[i-1] < A[i] > A[i+1] > ... > A[A.length - 1].

Example 1:

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

Example 2:

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

Note:

1. 3 <= A.length <= 10000
2. 0 <= A[i] <= 10^6
3. A is a mountain, as defined above.

Solution 1. Linear Search

// OJ: https://leetcode.com/problems/peak-index-in-a-mountain-array/
// Author: github.com/lzl124631x
// Time: O(N)
// Space: O(1)
class Solution {
public:
    int peakIndexInMountainArray(vector<int>& A) {
        int i = 1;
        while (i < A.size() && A[i] > A[i - 1]) ++i;
        return i - 1;
    }
};

Solution 2. Binary Search

// OJ: https://leetcode.com/problems/peak-index-in-a-mountain-array/
// Author: github.com/lzl124631x
// Time: O(logN)
// Space: O(1)
class Solution {
public:
    int peakIndexInMountainArray(vector<int>& A) {
        int L = 1, R = A.size() - 2;
        while (L <= R) {
            int M = (L + R) / 2;
            if (A[M] > A[M - 1]) L = M + 1;
            else R = M - 1;
        }
        return R;
    }
};