LeetCode:74.数组中的第K个最大元素

目录

1.数组中的第K个最大元素

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1.数组中的第K个最大元素

这道题通过堆排序解决，我们建立一个最大堆，将堆顶的K - 1个元素删除，留下的就是第K大的

class Solution {
public:
    void maxHeap(vector<int>& nums, int parent, int heapsize)
    {
        int left = parent * 2 + 1, right = parent * 2 + 2;
        int maxsum = parent;
        if(left < heapsize && nums[left] > nums[maxsum]) maxsum = left;
        if(right < heapsize && nums[right] > nums[maxsum]) maxsum = right;
        if(maxsum != parent)
        {
            swap(nums[maxsum], nums[parent]);
            maxHeap(nums, maxsum, heapsize);
        }
    }
    
    void buildMaxHeap(vector<int>& nums, int heapsize)
    {
        for(int i = heapsize; i >= 0; i--)
            maxHeap(nums, i, heapsize);
    }

    int findKthLargest(vector<int>& nums, int k) 
    {
        int heapsize = nums.size();
        buildMaxHeap(nums, heapsize);
        for(int i = nums.size() - 1; i >= nums.size() - k + 1; i--)
        {
            swap(nums[0], nums[i]);
            --heapsize;
            maxHeap(nums, 0, heapsize);
        }
        return nums[0];
    }
};