Article directory
LeetCode Brushing Notes-Data Structure-day20
215. The Kth Largest Element in an Array
1. Topic
Original title link: 215. The Kth largest element in an array
2. Problem-solving ideas
Direct set of quick row templates:
void quick_sort(int q[], int l, int r)
{
if (l >= r) return;
int i = l - 1, j = r + 1, x = q[l + r >> 1];
while (i < j)
{
do i ++ ; while (q[i] < x);
do j -- ; while (q[j] > x);
if (i < j) swap(q[i], q[j]);
}
quick_sort(q, l, j), quick_sort(q, j + 1, r);
}
3. Code
class Solution {
public:
int quickSort(vector<int>& nums,int l,int r,int k){
if(l>=r) return nums[k];
int x=nums[l+r>>1],i=l-1,j=r+1;
while(i<j){
do i++; while(nums[i]>x);
do j--; while(nums[j]<x);
if(i<j) swap(nums[i],nums[j]);
}
if(k<=j) return quickSort(nums,l,j,k);
else return quickSort(nums,j+1,r,k);
}
int findKthLargest(vector<int>& nums, int k) {
return quickSort(nums,0,nums.size()-1,k-1);
}
};
347. Top K High Frequency Elements
1. Description of the topic
Original title link: 347. Top K high-frequency elements
2. Problem-solving ideas
algorithm:哈希表+计数排序
Implementation:
- First use the hash table to count the number of occurrences of all numbers
- Use counting sorting to count the lower bound of the first k elements with the most occurrences
- Finally, traverse the hash table and add the number whose occurrences are greater than the lower bound to the answer
3. Code
class Solution {
public:
vector<int> topKFrequent(vector<int>& nums, int k) {
int n=nums.size();
map<int,int> hash;
for(auto x:nums) hash[x]++;
vector<int> s(n+1);
for(auto [a,b]:hash) s[b]++;
int i=n,t=0;
while(t<k) t+=s[i--];
vector<int> res;
for(auto [a,b]:hash){
if(b>i) res.push_back(a);
}
return res;
}
};
