C++ 代码

class Solution {
public:
    int findPeakElement(vector<int>& nums) {
        if(nums.empty()) return -1;
        int n = nums.size();
        if(n == 1) return 0;
        // n >= 2
        if(check_right(0, nums)) return 0;
        if(check_left(n - 1, nums)) return n - 1;
        // 此时 n 至少为 3
        int left = 1, right = n - 2; // n = 3 时, left = 1, right = 1 必进循环
        while(left <= right)
        {
            int mid = left + (right - left) / 2;
            if(check_left(mid, nums))
            {
                if(check_right(mid, nums))
                    return mid;
                else
                    left = mid + 1;
            }
            else
                right = mid - 1;
        }
        return -1;
    }

private:
    bool check_left(int index, vector<int> &nums)
    {
        // 查 index 左边的
        // index : [1 .. n - 1]
        if(nums[index - 1] < nums[index])
            return true;
        else
            return false;
    }
    bool check_right(int index, vector<int> &nums)
    {
        // 查 index 右边的
        // index : [0 .. n - 2]
        if(nums[index] > nums[index + 1])
            return true;
        else
            return false;
    }
};