1)利用快排思路求解~
static int partation(vector<int>& v, int l, int r) {
if (l > r) return -1;
if (l == r) return l;
int pivot = v[l];
while (l < r) {
while (l < r && v[r] >= pivot) r--;
swap(v[l], v[r]);
while (l < r && v[l] <= pivot) l++;
swap(v[l], v[r]);
}
v[l] = pivot;
return l;
}
static vector<int> getTopK(vector<int>& v, int k) {
int n = v.size();
vector<int> res;
int l = 0, r = n - 1;
int index = partation(v, l, r);
while (index != k - 1) {
if (index > k - 1) r = index - 1;
else if (index < k - 1) l = index + 1;
index = partation(v, l, r);
}
for (int i = 0; i < k; i++) {
res.push_back(v[i]);
}
return res;
}
2)利用堆排序思想求解~
具體思路是根據topk問題類型來确定建立大頂堆和小頂堆,然後對剩餘的k+1~n的對比目前元素和堆頂元素大小進行判斷進而調整得到最後的元素。
static void heapAdjust(vector<int>& v, int index, int n) {
int k = index, l = 2 * index + 1, r = 2 * index + 2;
if (l < n && v[l] < v[k]) k = l;
if (r < n && v[r] < v[k]) k = r;
if (k != index) {
swap(v[k], v[index]);
heapAdjust(v, k, n);
}
}
static void buildMinHeap(vector<int>& v, int n) {
int i = n / 2 - 1;
for (; i >= 0; i--) {
heapAdjust(v, i, n);
}
}
static void heapSort(vector<int>& v, int n) {
buildMinHeap(v, n);
for (int i = n - 1; i >= 0; i--) {
swap(v[0], v[i]);
heapAdjust(v, 0, i);
}
}
static vector<int> getRes(vector<int>& v,int k) {
int n = v.size();
vector<int> tmp(k);
for (int i = 0; i < k; i++) {
tmp[i] = v[i];
}
buildMinHeap(tmp, k);
for (int i = k; i < n; i++) {
if (v[i] > tmp[0]) {
tmp[0] = v[i];
heapAdjust(tmp, 0, k);
}
}
sort(tmp.begin(),tmp.end());
return tmp;
}
3)利用優先隊列求解(底層實作其實還是大頂堆或者小頂堆)
struct cmp1 {
bool operator()(int a,int b){
return a > b;
}
};
vector<int> getTopK(vector<int>& v, int k) {
int n = v.size();
priority_queue<int, vector<int>, cmp1> q;
for (int i = 0; i < n; i++) {
if (q.size() < k) {
q.push(v[i]);
}
else {
if (v[i] > q.top()) {
q.pop();
q.push(v[i]);
}
}
}
vector<int> res;
while (!q.empty()) {
int tmp = q.top();
q.pop();
res.push_back(tmp);
}
return res;
}