leetcode

77.组合

力扣链接

class Solution {
public:
    std::vector<vector<int>> res;
    std::vector<int> path;
    void backtracking(int n, int k, int startidx){
        if (path.size()==k){
            res.push_back(path);
            return;
        }
        for (int i = startidx; i <= n; i++){
            path.push_back(i);
            backtracking(n, k, i+1);
            // 注意pop.back()空括号 错误path.pop_back(i)
            path.pop_back();
        }
    }

    std::vector<vector<int>> combine(int n, int k) {
        backtracking(n, k, 1);
        return res;
    }
};

剪枝

class Solution {
public:
    std::vector<vector<int>> res;
    std::vector<int> path;
    void backtracking(int n, int k, int startidx){
        if (path.size()==k){
            res.push_back(path);
            return;
        }
        // i <= 剪枝
        for (int i = startidx; i <= n-(k-path.size())+1; i++){
            path.push_back(i);
            backtracking(n, k, i+1);
            // 注意pop.back()空括号 错误path.pop_back(i)
            path.pop_back();
        }
    }

    std::vector<vector<int>> combine(int n, int k) {
        backtracking(n, k, 1);
        return res;
    }
};