[LeetCode] 77. Combinations 全组合
Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.
For example,
If n = 4 and k = 2, a solution is:
[ [2,4], [3,4], [2,3], [1,2], [1,3], [1,4], ]
给两个数字n, k，返回所有由[1...n]中的k数字组合的可能解。
解法1: 递归
解法2: 迭代
C++: Recursion
class Solution {
public:
vector<vector<int>> combine(int n, int k) {
vector<vector<int>> res;
vector<int> out;
helper(n, k, 1, out, res);
return res;
}
void helper(int n, int k, int level, vector<int>& out, vector<vector<int>>& res) {
if (out.size() == k) res.push_back(out);
for (int i = level; i <= n; ++i) {
out.push_back(i);
helper(n, k, i + 1, out, res);
out.pop_back();
}
}
};
C++: Iteration
class Solution {
public:
vector<vector<int> > permute(vector<int> &num) {
vector<vector<int> > res;
permuteDFS(num, 0, res);
return res;
}
void permuteDFS(vector<int> &num, int start, vector<vector<int> > &res) {
if (start >= num.size()) res.push_back(num);
for (int i = start; i < num.size(); ++i) {
swap(num[start], num[i]);
permuteDFS(num, start + 1, res);
swap(num[start], num[i]);
}
}
};
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[LeetCode] 17. Letter Combinations of a Phone Number 电话号码的字母组合
