Find Peak Element —— Leetcode

BlitzSkies

于 2015-09-05 15:46:36 发布

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本文介绍了一种使用二分查找法寻找数组中峰值元素的方法。峰值元素是指比其邻居大的元素。文章提供了一个C++实现的例子，该算法适用于没有重复元素的数组，并能够在O(log n)的时间复杂度内找到一个峰值的索引。

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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.

二分法找峰值，代码中有一个注意点。

class Solution {
public:

    int binarySearch(vector<int> &nums, int low, int high) {
        if(low == high)
            return low;
            
        int mid1 = (low+high)/2;
        int mid2 = mid1+1;
        
        if(nums[mid1] < nums[mid2])
            return binarySearch(nums, mid2, high);  //此处若换成mid1则错误，应该选择高的。
        else
            return binarySearch(nums, low, mid1);
    }

    int findPeakElement(vector<int>& nums) {
        return binarySearch(nums, 0, nums.size()-1);
    }
};