05 August 2008
Given two integers n and k, return all possible combinations of k numbers out of 1 … n.
You may return the answer in any order.
Example 1:
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Input: n = 4, k = 2
Output:
[
[2,4],
[3,4],
[2,3],
[1,2],
[1,3],
[1,4],
]
Example 2:
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2
Input: n = 1, k = 1
Output: [[1]]
Constraints:
利用数学归纳法的递归写法，和回溯模板。
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class Solution {
public List<List<Integer>> combine(int n, int k) {
List<List<Integer>> results = new ArrayList<>();
combine(results, new ArrayList<Integer>(), n, k, 1);
return results;
}
private void combine(List<List<Integer>> results, List<Integer> oneResult, int n, int k, int index) {
if (k == 0) {
results.add(new ArrayList<>(oneResult));
return;
}
for (int i = index; i <= n; i++) {
oneResult.add(i);
combine(results, oneResult, n, k - 1, i + 1);
oneResult.remove(oneResult.size() - 1);
}
}
}