leetcode 216: Combination Sum III

Combination Sum III

Total Accepted: 1747 Total Submissions: 6301

Find all possible combinations of k numbers that add up to a number n, given that only numbers from 1 to 9 can be used and each combination should be a unique set of numbers.

Ensure that numbers within the set are sorted in ascending order.

Example 1:

Input: k = 3, n = 7

Output:

[[1,2,4]]

Example 2:

Input: k = 3, n = 9

Output:

[[1,2,6], [1,3,5], [2,3,4]]

[思路]

因为k是不定的, 所以无法用LOOP. 递归是此类题的常用解法. 为了方便, 多用一个variable: sum保存当前cur中所有数的和. 注意要新建一个list 放入结果中, 否则放入的reference 会指向原来的不断变化的list,    res.add(new ArrayList(cur));

[CODE]

public class Solution {
    //k=2  n=3
    public List<List<Integer>> combinationSum3(int k, int n) {
        List<List<Integer>> res = new ArrayList<List<Integer>>();
        if(k<1 || n<1) return res;
        List<Integer> cur = new ArrayList<Integer>();
        rec(res, cur, 0, k, n, 1);
        return res;
    }
    
    private void rec(List<List<Integer>> res, List<Integer> cur, int sum, int k, int n, int level) {
        if(sum==n && k==0) {
            res.add(new ArrayList(cur));
            return;
        } else if(sum>n || k<=0) return;
        
        for(int i=level; i<=9; i++) {
            cur.add(i);
            rec(res, cur, sum+i, k-1, n, i+1);
            cur.remove(cur.size() - 1);
        }
    }
}