回溯：i宽度约束，sum深度约束

func combinationSum3(k int, n int) [][]int {res := [][]int{}path := []int{}var help func(startIndex, sum, k, n int)help = func(startIndex, sum, k, n int) {if len(path) == k {if sum == n {tmp := make([]int, k)copy(tmp, path)res = append(res, tmp)}return}for i := startIndex; i <= 9 - (k - len(path)) + 1; i++ {path = append(path, i)help(i + 1, sum + i, k, n)path = path[:len(path) - 1]}}help(1, 0, k, n)return res
}

回溯：path用来处理路径上的结果参数；递归函数用来寻找可能符合要求的路径 ；回溯是为了保证路线之间的独立性

func letterCombinations(digits string) []string {res := []string{}if len(digits) == 0 {return res}var path []bytephoneMap := map[string]string{"2": "abc","3": "def","4": "ghi","5": "jkl","6": "mno","7": "pqrs","8": "tuv","9": "wxyz",}var help func(index int, digits string)help = func(index int, digits string) {if len(path) == len(digits) {res = append(res, string(path))return}num := string(digits[index])if chars, ok := phoneMap[num]; ok {for i := 0; i < len(chars); i++ {path = append(path, []byte(chars)[i])help(index+1, digits)path = path[:len(path)-1]}}}help(0, digits)return res
}