254. Factor Combinations

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Numbers can be regarded as product of its factors. For example,

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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:
You may assume that n is always positive.
Factors should be greater than 1 and less than n.
Examples:
input: 1
output:

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[]

input: 37
output:

1
[]

input: 12
output:

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[
  [2, 6],
  [2, 2, 3],
  [3, 4]
]

input: 32
output:

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[
  [2, 16],
  [2, 2, 8],
  [2, 2, 2, 4],
  [2, 2, 2, 2, 2],
  [2, 4, 4],
  [4, 8]
]

解法1： Backtracking

lang: java
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class Solution {
    public List<List<Integer>> getFactors(int n) {
        List<List<Integer>> res = new ArrayList<>();
        List<Integer> current = new ArrayList<>();
        if (n <= 2) return res;
        helper(n, 2, current, res);
        return res;
    }
    private void helper(int n, int start, List<Integer> current, List<List<Integer>> res) {
        if (n <= 0) {
            return;
        }
        if (n == 1) {
            if (current.size() > 1) {
                res.add(new ArrayList<Integer>(current));
                return;
            }
        }
        for (int i = start; i <= n; i++) {
            if (n % i == 0) {
                current.add(i);
                helper(n / i, i, current, res);
                current.remove(current.size() - 1);
            }
        }
        return;
    }
}