LeetCode 852. Peak Index in a Mountain Array（C++）

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.

二分法：

class Solution{ 
public:
        int peakIndexInMountainArray(vector<int>& a){ 
            int beg = 1,end = a.size();
            int mid = (beg + end) / 2;

            while(beg <= end){ 
                if(a[mid] < a[mid - 1]){ 
                    end = mid - 1;
                }
                else if(a[mid] < a[mid + 1]){ 
                    beg = mid + 1;
                }
                else{ 
                    break;
                }
                mid = (beg + end) / 2;
            }
            return mid;
        }
};

最大值法

class Solution{ 
public:
        int peakIndexInMountainArray(vector<int>& a){ 
            int max_elem = *max_element(a.begin(), a.end());
            int pos;
            for(pos = 0; pos < a.size(); ++pos){
                if(a[pos] == max_elem)
                    break;
            }
            
            return pos;
        }
};