77 Combinations

77. Combinations​
1. Question
Given two integersnandk, return all possible combinations ofknumbers out of 1 ...n.
For example,
Ifn= 4 and k= 2, a solution is:
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[
2
  [2,4],
3
  [3,4],
4
  [2,3],
5
  [1,2],
6
  [1,3],
7
  [1,4],
8
]
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2. Implementation
(1) Backtracking
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class Solution {
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    public List<List<Integer>> combine(int n, int k) {
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        List<List<Integer>> res = new ArrayList<>();
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        List<Integer> combination = new ArrayList<>();
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        getCombinations(1, n, k, combination, 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 n, int k, List<Integer> combination, List<List<Integer>> res) {
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        if (k == 0) {
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            res.add(new ArrayList<>(combination));
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            return;
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        }
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​
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        for (int i = start; i <= n; i++) {
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            combination.add(i);
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            getCombinations(i + 1, n, k - 1, combination, res);
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            combination.remove(combination.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(C(n,k)), 空间复杂度O(C(n, k))

60 Permutation Sequence

78 Subsets

Last modified 2yr ago

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Contents

77. Combinations

1. Question

2. Implementation

3. Time & Space Complexity

77. Combinations​

1. Question

2. Implementation

3. Time & Space Complexity

77. Combinations