LeetCode 77. Combinations

[Woodyiiiiiii]

Given a collection of distinct integers, return all possible permutations.

Example:

Input: [1,2,3]
Output:
[
  [1,2,3],
  [1,3,2],
  [2,1,3],
  [2,3,1],
  [3,1,2],
  [3,2,1]
]

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这题跟46. Permutations很像，使用DFS＋回溯法的思想来解。不同的是or循环的初始条件。

class Solution {
    public List<List<Integer>> combine(int n, int k) {
        List<List<Integer>> res = new LinkedList<>();
        if (n < 1 || k < 1 || n < k) return res;
        List<Integer> tmp = new LinkedList<>();
        dfs(res, tmp, n, k, 1);
        return res;
    }
    public void dfs(List<List<Integer>> res, List<Integer> tmp, int n, int k, int start) {
        if (tmp.size() == k) {
            res.add(new LinkedList<>(tmp));
            return;
        }
        for (int i = start; i <= n; ++i) {
            tmp.add(i);
            dfs(res, tmp, n, k, i + 1);
            tmp.remove(tmp.size() - 1);
        }
    }
}

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相似题目：

• 46. Permutations
• 78. Subsets

参考资料：

• https://leetcode.com/problems/combinations/