LeetCode Online Judge 题目C# 练习 - PermutationsII

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].

public static List<List<int>> PermutationsII(List<int> num)
{
List<List<int>> ret = new List<List<int>>();

if (num.Count == 0)
return ret;
if (num.Count == 1)
{
ret.Add(num);
return ret;
}

num.Sort();

ret.Add(new List<int>(num));
while(NextPermutationII(num))
ret.Add(new List<int>(num));

return ret;
}

public static bool NextPermutationII(List<int> num)
{
if (num.Count <= 1)
return false;

//find the falling edge
int edge = -1;
for (int i = num.Count - 2; i >= 0; i--)
{
if (num[i] < num[i + 1])
{
edge = i;
break;
}
}

if (edge > -1)
{
//find replacement
for (int i = edge + 1; i < num.Count; i++)
{
if (num[edge] >= num[i])
{
NextPermutationSwap(num, edge, i - 1);
break;
}
if (i == num.Count - 1)
{
NextPermutationSwap(num, edge, i);
break;
}
}
}
else
{
return false;
}

for (int i = edge + 1, j = num.Count - 1; i <= edge + (num.Count - edge - 1) / 2; i++, j--)
{
NextPermutationSwap(num, i, j);
}
return true;
}

代码分析：

　　先把输入数组排列一下，利用之前做的一题，找下一个permutation的那一题，改写一下函数，如果找到返回true，然后再加入到结果里面。