852. Peak Index in a Mountain Array

Problem:

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.

思路：

Solution (C++):

int peakIndexInMountainArray(vector<int>& A) {
    int low = 0, high = A.size()-1;
    while (low <= high) {
        int mid = low + (high-low) / 2;
        if (A[mid] > A[mid+1] && A[mid] > A[mid-1])  return mid;
        else if (A[mid] < A[mid+1]) low = mid + 1;
        else  high = mid-1;
    }
    return -1;
}

性能：

Runtime: 16 ms  Memory Usage: 7.1 MB

思路：

Solution (C++):

int peakIndexInMountainArray(vector<int>& A) {
    for (int i = 0; i < A.size()-1; ++i) {
        if (A[i] > A[i+1])  return i;
    }
    return -1;
}

性能：

Runtime: 12 ms  Memory Usage: 7.4 MB