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77.combinations.cpp
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52 lines (48 loc) · 1.21 KB
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// Tag: Backtracking
// Time: O(k * C(n, k))
// Space: O(K)
// Ref: -
// Note: -
// Given two integers n and k, return all possible combinations of k numbers chosen from the range [1, n].
// You may return the answer in any order.
//
// Example 1:
//
// Input: n = 4, k = 2
// Output: [[1,2],[1,3],[1,4],[2,3],[2,4],[3,4]]
// Explanation: There are 4 choose 2 = 6 total combinations.
// Note that combinations are unordered, i.e., [1,2] and [2,1] are considered to be the same combination.
//
// Example 2:
//
// Input: n = 1, k = 1
// Output: [[1]]
// Explanation: There is 1 choose 1 = 1 total combination.
//
//
// Constraints:
//
// 1 <= n <= 20
// 1 <= k <= n
//
//
class Solution {
public:
vector<vector<int>> combine(int n, int k) {
vector<vector<int>> res;
vector<int> tmp;
dfs(n, k, 1, tmp, res);
return res;
}
void dfs(int n, int k, int start, vector<int> &tmp, vector<vector<int>> &ans) {
if (tmp.size() == k) {
ans.push_back(tmp);
return;
}
for (int i = start; i <= n; i++) {
tmp.push_back(i);
dfs(n, k, i + 1, tmp, ans);
tmp.pop_back();
}
}
};