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M

Combination DFS。 画个图想想. 每次从1~n里面pick一个数字i

因为下一层不能重新回去 [0~i]选，所以下一层recursive要从i+1开始选。

```

/*

Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.

Example

For example,

If n = 4 and k = 2, a solution is:

[[2,4],[3,4],[2,3],[1,2],[1,3],[1,4]]

Tags Expand

Backtracking Array

Thinking process:

Use a helper method to perform recursive backtracking:add an element to next-level recursive call, and remote the entry after the recursive call.

Note: When 'new' something, cannot use 'List' because it's a abstract class. Need to new 'ArrayList'

*/

public class Solution {

public List<List<Integer>> combine(int n, int k) {

List<List<Integer>> rst = new ArrayList<List<Integer>>();

if (n <= 0 || k <= 0) {

return rst;

}

List<Integer> solution = new ArrayList<Integer>();

helper(rst, solution, n, k, 1);// Start == 1 because we want 1 ~ n in this problem

return rst;

}

public void helper(List<List<Integer>> rst,

List<Integer> solution, int n, int k, int start) {

if (solution.size() == k) {

rst.add(new ArrayList(solution));

return;

}

for (int i = start; i <= n; i++) {// <=n because we want 1 ~ n in this problem

solution.add(i);

helper(rst, solution, n, k, i + 1);

solution.remove(solution.size() - 1); //Back-track

}

}

}

```