面试c++手撕代码题

 快排

#include 
#include 
using namespace std;

int partition(vector& arr, int left, int right){
	int pivot = arr[left];
	int j = left;
	for (int i = left + 1; i <= right; i++) { //保留第一个pivot位置不变，往后面遍历，遇到一个小于pivot的就交换
		if (arr[i] < pivot) {
			int temp = arr[i];
			arr[i] = arr[j + 1];
			arr[j + 1] = temp;
			j++;
		}
	}
	arr[left] = arr[j];
	arr[j] = pivot;
	return j;
}

void quicksort(vector& arr, int left, int right) {
	if (left < right) { //这里是递归的终止条件。
		int mid = partition(arr, left, right);
		quicksort(arr, left, mid - 1);
		quicksort(arr, mid + 1, right);
	}
}

int main() {
	vector arr = { 1, 2, 5, 7, 3, 5, 9 };
	quicksort(arr, 0, arr.size() - 1);
	for (int i = 0; i < arr.size(); i++) {
		cout << arr[i] << endl;
	}
	return 0;
}

topk 

#include 
#include 
#include 
using namespace std;

//topk最大的k个用小顶堆，greater函数。或者自定义类 bool operator()(const........)
//topk最小的k个用大顶堆，默认即可。或者less，或者自定义
void topfrequent(vector& arr, int k) {
	unordered_map map;
	for (auto i : arr) {
		map[i]++;
	}
	priority_queue, vector>, greater>> heap;
	int num = 0;
	for (auto i : map) {
		heap.push({ i.second, i.first });
		num++;
		if (num > k) {
			heap.pop();
		}
	}
	for (int i = 0; i < k; i++) {
		cout << heap.top().second << endl;
		heap.pop();
	}
}

int main() {
	vector arr = { 1,1,1,2,2,3 };
	topfrequent(arr, 2);
	return 0;
}

NMS

#include 
#include 
#include 
using namespace std;

struct BoundingBox {
	float x1, y1, x2, y2;
	float confidence;
};

//bool cmp(const BoundingBox& a1, const BoundingBox& a2) {
//	return a1.confidence > a2.confidence;
//}
class MyCompare {
public:
	bool operator() (const BoundingBox& a1, const BoundingBox& a2) {
		return a1.confidence > a2.confidence;
	}
};

float calculate_iou(const BoundingBox& box1, const BoundingBox& box2) {
	float heigh = max(0.f, min(box1.y2, box2.y2) - max(box1.y1, box2.y1));
	float width = max(0.f, min(box1.x2, box2.x2) - max(box1.x1, box2.x1));
	float iou = (heigh * width) / ((box1.x2 - box1.x1) * (box1.y2 - box1.y1) + (box2.x2 - box2.x1) * (box2.y2 - box2.y1) - heigh * width);
	return iou;
}

vector NMS(vector& boxes, float score_threshold) {
	sort(boxes.begin(), boxes.end(), MyCompare());
	vector result;
	for (auto& box : boxes) {
		bool keep = true;
		for (auto& keep_box : result) {
			float iou = calculate_iou(keep_box, box);
			if (iou > score_threshold) {
				keep = false;
				break;
			}
		}
		if (keep) {
			result.push_back(box);
		}
	}
	return result;
}


int main() {
	std::vector detections = {
	{10, 10, 50, 50, 0.9},   // 框1：左上角(10, 10)，右下角(50, 50)，置信度0.9
	{20, 20, 60, 60, 0.85}   // 框2：左上角(20, 20)，右下角(60, 60)，置信度0.85
	};

	float iouThreshold = 0.1;
	std::vector filteredBoxes = NMS(detections, iouThreshold);

	// 预期输出：只保留置信度更高的框1
	// 输出结果的顺序可能会有所不同
	for (const auto& box : filteredBoxes) {
		std::cout << "Left: " << box.x1 << ", Top: " << box.y1 << ", Right: " << box.x2 << ", Bottom: "
			<< box.y2 << ", Confidence: " << box.confidence << std::endl;
	}
}