1.题目： 

 

 

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2.解析：

 

 

代码：

public int peakIndexInMountainArray(int[] arr) {
        int left = 1, right = arr.length-2;

        while(left < right) {
            int mid = left + (right-left+1) / 2;
            if(arr[mid] > arr[mid-1]) left = mid;
            else  right = mid-1;
        }
        return left;
    }