Find Peak Number

Question

A peak element is an element that is greater than its neighbors. Given an input array where num[i] ≠ num[i+1], find a peak element and return its index. The array may contain multiple peaks, in that case return the index to any one of the peaks is fine. You may imagine that num[-1] = num[n] = -∞. For example, in array [1, 2, 3, 1], 3 is a peak element and your function should return the index number 2.

Similar Question

Maximum number in mountain sequence

Analysis

• 从要抛去哪一半去思考
  • 分析单调上升/下降的情况即可知道如何抛弃
• corner case
  • 只有一个数、只有两个数、越界了？
  • just compare nums[start], nums[end]

Answer

class Solution {
    public int findPeakElement(int[] nums) {
        if(nums == null || nums.length == 0) return -1;

        int start = 0; 
        int end = nums.length - 1;
        int mid;

        while (start + 1 < end) {
            mid = start + (end - start) / 2;

            if (nums[mid] < nums[mid+1]) start = mid;
            else end = mid;
        }

       if(nums[start] > nums[end]) return start;
        else return end;

    }
}