Day 94, ##, Find Peak Element

Find Peak Element

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.

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class Solution {
public:
    int findPeakElement(const vector<int> &num) {
        if (num[0] > num[1]) return 0;
        if (num[num.size() - 1] > num[num.size() - 2]) return num.size() - 1; 
        
        int left = 1, right = num.size() - 2;
        while (left <= right) {
            int mid = right + (left - right) / 2;
            if (num[mid] > num[mid - 1] && num[mid] > num[mid + 1]) {
                return mid;
            }else if (num[mid] > num[mid + 1]) {
                right = mid - 1;
            }else {
                left = mid + 1;
            }
        }
        
        return num.size() - 1;
    }
};

Java, updated on Oct-10th-2018
1. left < right 保证了mid + 1时不用对mid做boundry check
2. right = mid, 因为此时[mid] > [mid + 1], mid还算是一个candidate
3. ToDo: 二分法的一种写法是不设base case，然后由while里的条件来结束，最后返回left。https://shibaili.blogspot.com/2018/10/658-find-k-closest-elements.html 第2个方法用了类似的方法

class Solution {
    public int findPeakElement(int[] nums) {
        int left = 0, right = nums.length - 1;
        
        while (left < right) {
            int mid = (left + right) / 2;
            if (nums[mid] < nums[mid + 1]) {
                left = mid + 1;
            }else {
                right = mid;
            }
        }
        
        return left;
    }
}