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);
}
}
}