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  • 852. Peak Index in a Mountain Array

    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.

    Approach #1: Binary Search.

    class Solution {
    public:
        int peakIndexInMountainArray(vector<int>& A) {
            int n = A.size();
            int l = 0; 
            int r = n-1;
            while (l <= r) {
                int m = l + (r - l) / 2;
                if (A[m-1] < A[m] && A[m] > A[m+1]) return m;
                else if (A[m-1] < A[m] && A[m+1] > A[m]) l = m + 1;
                else r = m - 1;
            }
            return -1;
        }
    };
    
    Runtime: 12 ms, faster than 53.74% of C++ online submissions for Peak Index in a Mountain Array.
    永远渴望,大智若愚(stay hungry, stay foolish)
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  • 原文地址:https://www.cnblogs.com/h-hkai/p/9937235.html
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