Numbers can be regarded as product of its factors. For example,
8 = 2 x 2 x 2; = 2 x 4.Write a function that takes an integer n and return all possible combinations of its factors.
Note:
Each combination's factors must be sorted ascending, for example: The factors of 2 and 6 is [2, 6], not [6, 2].You may assume that n is always positive.Factors should be greater than 1 and less than n.
Examples: input: 1output:
[] input: 37 output: [] input: 12 output: [ [2, 6], [2, 2, 3], [3, 4] ] input: 32 output: [ [2, 16], [2, 2, 8], [2, 2, 2, 4], [2, 2, 2, 2, 2], [2, 4, 4], [4, 8] ]快速解法： public class Solution { public List<List<Integer>> getFactors(int n) { List<List<Integer>> result = new ArrayList<List<Integer>>(); helper(result, new ArrayList<Integer>(), n, -1); return result; } public void helper(List<List<Integer>> result, List<Integer> item, int n, int start){ if (start != -1) { item.add(n); result.add(new ArrayList<Integer>(item)); item.remove(item.size() - 1); } for (int i = Math.max(start,2); i <= Math.sqrt(n); ++i) { if (n % i == 0) { item.add(i); helper(result, item, n/i, i); item.remove(item.size()-1); } } } }请忽略一下解法什么破方法慢死！recursion的解法： public class Solution { public List<List<Integer>> getFactors(int n) { List<List<Integer>> result = new ArrayList<List<Integer>>(); helper(result, new ArrayList<Integer>(), n, 2); return result; } public void helper(List<List<Integer>> result, List<Integer> item, int n, int start){ if (n <= 1) { if (item.size() > 1) { result.add(new ArrayList<Integer>(item)); System.out.println("add item to result"); } return; } for (int i = start; i <= n; ++i) { if (n % i == 0) { item.add(i); System.out.println("item"); System.out.println(item); helper(result, item, n/i, i); item.remove(item.size()-1); System.out.println("removed item"); System.out.println(item); } } } }
运行结果：
test case n=8
item [2] item [2, 2] item [2, 2, 2] add to result removed item [2, 2] removed item [2] item [2, 4] add to result removed item [2] removed item [] item [4] removed item [] item [8] removed item []
结果：
Your answer [[2,2,2],[2,4]] Expected answer [[2,4],[2,2,2]]
reference：https://leetcode.com/discuss/51250/my-recursive-dfs-java-solution
转载于:https://www.cnblogs.com/hygeia/p/5087613.html
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