题目:
Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.
Example:
Input: n = 4, k = 2
Output:
[
[2,4],
[3,4],
[2,3],
[1,2],
[1,3],
[1,4],
]
代码:
方法1——
class Solution {
public:
vector<vector<int>> combine(int n, int k) {
vector<vector<int> > res;
if (k > n || k <= 0)return res;
if (n == k) {
vector<int> v;
for (int i = 1; i <= n; i++) {
v.push_back(i);
}
res.push_back(v);
}
else {
if (k == 1) {
for (int i = 1; i <= n; i++) {
vector<int> v;
v.push_back(i);
res.push_back(v);
}
}
else {
vector<vector<int> >v1 = combine(n - 1, k);
vector<vector<int> >v2 = combine(n - 1, k - 1);
for (int i = 0; i < v1.size(); i++) {
res.push_back(v1[i]);
}
for (int i = 0; i < v2.size(); i++) {
v2[i].push_back(n);
res.push_back(v2[i]);
}
}
}
return res;
}
};
方法二(dfs)——
class Solution {
private:
vector< vector<int> > result;
vector<int> v;
void dfs(int i,int n,int k) {
while (i <= n) {
v[v.size() - k] = i++;
if (k > 1)
dfs(i, n, k - 1);
else
result.push_back(v);
}
}
public:
vector<vector<int>> combine(int n, int k) {
v.resize(k);
dfs(1, n, k);
return result;
}
};