p77

LeetCode p77 Combinations 题解

1.题目：

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],
]

题意：

从n个数中选出k个数，顺序排列输出所有的组合。

2.解题思路:

见代码，类似搜索，去掉多余的分支。

3.代码

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[title] [] [url] [link text]

 
public class Solution {
    public List<List<Integer>> combine(int n, int k) {
        List<List<Integer>> ansList = new ArrayList<List<Integer>>();
        List<Integer> list = new ArrayList<Integer>();
		helper(n, k, ansList, 1, list);
		return ansList;
	}

	public void helper(int n, int k, List<List<Integer>> ansList, int a,
			List<Integer> list) {

		if (k == list.size()) {
			List<Integer> cIntegers = new ArrayList<Integer>(list);
			ansList.add(cIntegers);
			//System.out.println(Arrays.toString(cIntegers.toArray()));
			// list.remove(list.size() - 1);
			return;
		}
		for (int i = a; i <= n; i++) {
			if (list.size() + n - i + 1 < k) {
				break;
			}
			list.add(i);
			helper(n, k, ansList, i + 1, list);
			list.remove(list.size() - 1);
		}
	}
}

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4.一些总结：