Interview Classic 150 Questions P77 Combination

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Timeline

Timeline

2025-11-28

init

Backtracking

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Problem:

P77 Combination

https://leetcode.cn/problems/combinations/description/?envType=study-plan-v2&envId=top-interview-150

Note the next starts frompath.back()+1Starting this way avoids duplicates like (1,2),(2,1).

#include <vector>
using std::vector;

class Solution {
private:
	vector<vector<int> > res;
	void backtrace(vector<int> &path, int n, int k)
	{
		if (path.size() == k) {
			res.push_back(path);
			return;
		}
		int start = path.empty() ? 1 : path.back() + 1;
		for (int i = start; i <= n; i++) {
			path.push_back(i);

			backtrace(path, n, k);
			path.pop_back();
		}
	}
public:
	vector<vector<int> > combine(int n, int k)
	{
		res = vector<vector<int> >();
		vector<int> path;
		backtrace(path, n, k);
		return res;
	}
};