leetcode 216: Combination Sum III，leetcodetwosum

leetcode 216: Combination Sum III，leetcodetwosum

Combination Sum IIITotal Accepted: 1747 Total Submissions: 6301

Find all possible combinations of k numbers that add up to a numbern, 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]

因為k是不定的, 所以無法用LOOP. 遞迴是此類題的常用解法. 為了方便, 多用一個variable: sum儲存當前cur中所有數的和. 注意要建立一個list 放入結果中, 否則放入的reference 會指向原來的不斷變化的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);        }    }}