思路

顺序遍历

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

二分查找

 public int peakIndexInMountainArray(int[] A) {
        int low = 0;
        int high = A.length-1;
        while(low<=high){
            int mid = low + ((high-low)>>1);
            if(A[mid]>A[mid+1]&&A[mid]>A[mid-1]){
                return mid;
            }else if(A[mid]<A[mid+1]&&A[mid]>A[mid-1])
                low = mid+1;
            else
                high = mid -1;
        }
        return -1;
    }