代码随想录算法训练营|day28

93.复原IP地址

ip地址有效需要满足：遍历完s，将其分为4段，每段数字有效性：范围在[0,255],且不含前导零
为避免重复取值，需要startIndex作为下次递归的初始位置。当遍历结束startIndex == len(s)，且ip段数为4 len(path) == 4加入结果，否则提前回溯。
如果当前字符串满足要求，递归调用下一层

func restoreIpAddresses(s string) []string {
	res := []string{}
	path := []string{}
	var help func(s string, startIndex int)
	help = func(s string, startIndex int) {
		if len(path) == 4 {
            if startIndex == len(s) {
			    res = append(res, strings.Join(path, "."))
            }
			return
		}
		for i := startIndex; i < len(s); i++ {
			str := s[startIndex : i+1]
			if isValid(str) {
				path = append(path, str)
				help(s, i+1)
				path = path[:len(path)-1]
			} else {
				break
			}
		}
	}
	help(s, 0)
	return res
}

func isValid(str string) bool {
	if len(str) != 1 && str[0] == '0' {
		return false
	}
	i, _ := strconv.Atoi(str)
	if i > 255 {
		return false
	}
	return true
}

78.子集

子集相当于找树的节点，只要遍历到一个节点就加入结果

func subsets(nums []int) [][]int {
	res := [][]int{}
	path := []int{}
	var help func(nums []int, startIndex int)
	help = func(nums []int, startIndex int) {
		tmp := make([]int,len(path))
		copy(tmp,path)
		res = append(res, tmp)
		for i := startIndex; i < len(nums); i++ {
			path = append(path, nums[i])
			help(nums, i+1)
			path = path[:len(path)-1]
		}
	}
	help(nums, 0)
	return res
}

90.子集II

相较于上题子集问题，数组有重复元素，要求结果集合不能重复，需要去重
先对数组进行排序，然后同层去重【否则，相当于当前元素要走前一个元素已经走过的路】
同层去重：used[i - 1] = false,说明是从前一个元素回溯回来的，若used[i - 1] = true说明是进入下一个递归取值

func subsetsWithDup(nums []int) [][]int {
	res := [][]int{}
	path := []int{}
	used := make([]bool, len(nums))
	sort.Ints(nums)
	var help func(nums []int, startIndex int)
	help = func(nums []int, startIndex int) {
		tmp := make([]int, len(path))
		copy(tmp, path)
		res = append(res, tmp)
		for i := startIndex; i < len(nums); i++ {
			if i > 0 && nums[i] == nums[i-1] && used[i-1] == false {
				continue
			}
			path = append(path, nums[i])
			used[i] = true
			help(nums, i+1)
			used[i] = false
			path = path[:len(path)-1]
		}
	}
	help(nums, 0)
	return res
}

代码随想录文章详解

93.复原IP地址
78.子集
90.子集II