LeetCode Permutations II

题目：

Given a collection of numbers that might contain duplicates, return all possible unique permutations.

For example,
[1,1,2] have the following unique permutations:
[1,1,2], [1,2,1], and [2,1,1].

class Solution {
public:
	vector<vector<int> > permuteUnique(vector<int> &num) {
		res.clear();
		memset(used, false, sizeof(used));
		sort(num.begin(), num.end());
		dfs(0, num.size(), num);
		return res;
	}

private:
	vector<vector<int> > res;
	bool used[100];
	int a[100];
	void dfs(int dep, int maxDep, vector<int> &num) {
		if (dep == maxDep) {
			vector<int> tt;
			for (int i = 0; i < maxDep; i++)
				tt.push_back(a[i]);
			res.push_back(tt);
			return;
		}
		for (int i = 0; i < maxDep; i++) {
			if (!used[i]) {
				if (i != 0 && num[i] == num[i - 1] && !used[i - 1])
					continue;
				used[i] = true;
				a[dep] = num[i];
				dfs(dep + 1, maxDep, num);
				used[i] = false;
			}
		}
	}
};