1. 子集2

这题需要先进行排序，和候选人那题类似。防止出现重复的子集。

func subsetsWithDup(nums []int) [][]int {
	path := make([]int, 0)
	res := make([][]int, 0)
    sort.Ints(nums)
	var dfs func(nums []int, start int)
	dfs = func(nums []int, start int) {
		res = append(res, append([]int(nil), path...)) //使用append创建path的副本

		for i := start; i < len(nums); i++ {
			if i != start && nums[i] == nums[i-1] {
				continue
			}
			path = append(path, nums[i])
			dfs(nums, i+1)
			path = path[:len(path)-1]
		}
	}
	dfs(nums, 0)
	return res
}

2. 递增子序列

func findSubsequences(nums []int) [][]int {
    res := make([][]int,0)
    path := make([]int,0)
    var dfs func(nums []int,start int)
    dfs = func(nums []int,start int){
        if len(path)>=2{
            res = append(res,append([]int(nil), path...))
        }
        used := make(map[int]bool,len(path))
        for i:=start;i<len(nums);i++{
            if used[nums[i]]{//去重
                continue
            }
            if len(path)==0 || nums[i]>=path[len(path)-1]{
                path = append(path,nums[i])
                used[nums[i]]=true
                dfs(nums,i+1)
                path = path[:len(path)-1]
            }
        }
    }
    dfs(nums,0)
    return res
}

3. 全排列

var (
    res [][]int
    path  []int
    st    []bool   // state的缩写
)
func permute(nums []int) [][]int {
    res, path = make([][]int, 0), make([]int, 0, len(nums))
    st = make([]bool, len(nums))
    dfs(nums, 0)
    return res
}

func dfs(nums []int, cur int) {
    if cur == len(nums) {
        tmp := make([]int, len(path))
        copy(tmp, path)
        res = append(res, tmp)
    }
    for i := 0; i < len(nums); i++ {
        if !st[i] {
            path = append(path, nums[i])
            st[i] = true
            dfs(nums, cur + 1)
            st[i] = false
            path = path[:len(path)-1]
        }
    }
}

4. 全排列2

需要注意：如何去重。先对数组进行排序，对于相同的字母，如果前面没选，则后面的页不能选。

var(
    path []int
    res [][]int
)
func permuteUnique(nums []int) [][]int {
    path = make([]int,0)
    res = make([][]int,0)
    used := make([]bool,len(nums))
    sort.Ints(nums)
    dfs(nums,used,0)
    return res
}

func dfs(nums []int,used []bool,count int){
    if count==len(nums){
        res=append(res,append([]int(nil),path...))
    }
    for i:=0;i<len(nums);i++{
        if i!=0 && nums[i]==nums[i-1] && used[i-1]==false{
            continue
        }
        
        if used[i]==false{
            path = append(path,nums[i])
            used[i]=true
            dfs(nums,used,count+1)
            path = path[:len(path)-1]
            used[i]=false
        }
    }
}