[LeetCode] 852. Peak Index in a Mountain Array

ispring 發布于2019-08-16 13:49 / 1042人閱讀

Problem

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:

3 <= A.length <= 10000
0 <= A[i] <= 10^6
A is a mountain, as defined above.

Solution

class Solution {
    public int peakIndexInMountainArray(int[] A) {
        int start = 0, end = A.length-1;
        while (start+2 < end) {
            int mid = start+(end-start)/2;
            if (mid-1 >= start && mid+1 <= end && A[mid-1] < A[mid] && A[mid] > A[mid+1]) {
                return mid;
            } else if (mid-1 >= start && mid+1 <= end && A[mid-1] < A[mid] && A[mid] < A[mid+1]) {
                start = mid;
            } else if (mid-1 >= start && mid+1 <= end && A[mid-1] > A[mid] && A[mid] > A[mid+1]) {
                end = mid;
            } else {
                break;
            }
        }
        return start+1;
    }
}

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