216 Combination Sum III

216. Combination Sum III​
1. Question
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.
Example 1:
Input: k= 3,n= 7
Output:
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[[1,2,4]]
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Example 2:
Input:k= 3,n= 9
Output:
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[[1,2,6], [1,3,5], [2,3,4]]
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2. Implementation
(1) Backtracking
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class Solution {
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    public List<List<Integer>> combinationSum3(int k, int n) {
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        List<List<Integer>> res = new ArrayList<>();
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        List<Integer> combinations = new ArrayList<>();
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        getCombinations(1, 0, k, n, combinations, res);
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        return res;
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    }
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​
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    public void getCombinations(int start, int sum, int k, int target, List<Integer> combinations, List<List<Integer>> res) {
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        if (sum > target) {
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            return;
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        }
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​
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        if (k == 0 && sum == target) {
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            res.add(new ArrayList<>(combinations));
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            return;
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        }
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​
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        for (int i = start; i <= 9; i++) {
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            combinations.add(i);
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            getCombinations(i + 1, sum + i, k - 1, target, combinations, res);
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            combinations.remove(combinations.size() - 1);
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        }
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    }
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}
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3. Time & Space Complexity
Backtracking: 时间复杂度O(n^k), 根据组合公式得到，空间复杂度O(n^k)

212 Word Search II

254 Factor Combinations

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Contents

216. Combination Sum III

1. Question

2. Implementation

3. Time & Space Complexity

216. Combination Sum III​

1. Question

2. Implementation

3. Time & Space Complexity

216. Combination Sum III