【CODE】Kth Largest Element in an Array

215. Kth Largest Element in an Array

Medium

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Find the kth largest element in an unsorted array. Note that it is the kth largest element in the sorted order, not the kth distinct element.

Example 1:

Input: [3,2,1,5,6,4]  and k = 2
Output: 5

Example 2:

Input: [3,2,3,1,2,4,5,5,6]  and k = 4
Output: 4

Note:
You may assume k is always valid, 1 ≤ k ≤ array's length.

class Solution {
public:
    int findKthLargest(vector<int>& nums, int k) {
        map<int,int> mp;
        for(int i=0;i<nums.size();i++){
            mp[nums[i]]++;
        }
        int tar=0;
        map<int,int>::iterator it=mp.end()--;
        for(;;it--){
            if(it->second+tar<k) tar+=it->second;
            else if(it->second+tar>=k) return it->first;
        }
        return mp.begin()->first;
    }
};

• 使用map做映射，从后向前遍历map。
• Runtime: 20 ms, faster than 34.02% of C++ online submissions for Kth Largest Element in an Array.
• Memory Usage: 11.5 MB, less than 6.06% of C++ online submissions for Kth Largest Element in an Array.
• Next challenges:
• Wiggle Sort II
• Kth Largest Element in a Stream
• K Closest Points to Origin
• https://www.cnblogs.com/grandyang/p/4539757.html
• 本题考查的应该是快速排序：！！！

class Solution {
public:
    int findKthLargest(vector<int>& nums,int k){
        int left=0,right=nums.size()-1;
        while(true){
            int pos=partition(nums,left,right);
            if(pos==k-1) return nums[pos];
            if(pos>k-1) right=pos-1;
            else left=pos+1;
        }
    }
    int partition(vector<int>& nums,int left,int right){
        int pivot=nums[left],l=left+1,r=right;
        while(l<=r){
            if(nums[l]<pivot && nums[r]>pivot){
                swap(nums[l++],nums[r--]);
            }
            if(nums[l]>=pivot) ++l;
            if(nums[r]<=pivot) --r;
        }
        swap(nums[left],nums[r]);
        return r;
    }
};

• Runtime: 80 ms, faster than 11.88% of C++ online submissions for Kth Largest Element in an Array.
• Memory Usage: 9.1 MB, less than 100.00% of C++ online submissions for Kth Largest Element in an Array.