> For the complete documentation index, see [llms.txt](https://a.b.cr/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://a.b.cr/dictionary/algorithm/diary/2022/05/2022-05-25.md). # 2022-05-25 ## [77. Combinations](https://leetcode.com/problems/combinations/) ### Description Given two integers `n` and `k`, return *all possible combinations of* `k` *numbers out of the range* `[1, n]`. You may return the answer in **any order**. **Example 1:** ``` Input: n = 4, k = 2 Output: [ [2,4], [3,4], [2,3], [1,2], [1,3], [1,4], ] ``` **Example 2:** ``` Input: n = 1, k = 1 Output: [[1]] ``` **Constraints:** * `1 <= n <= 20` * `1 <= k <= n` ### Solution #### Approach #0: DFS ```go func combine(n int, k int) [][]int { res := [][]int{} item := []int{} var dfs func(int) dfs = func(start int) { if len(item) == k { a := make([]int, k) copy(a, item) res = append(res, a) return } for i := start; i <= n; i++ { item = append(item, i) dfs(i + 1) item = item[:len(item)-1] } } dfs(1) return res } ``` #### Approach #1: Alphabetical Order ```go func combine(n int, k int) [][]int { var temp []int for i := 1; i <= k; i++ { temp = append(temp, i) } temp = append(temp, n+1) var res [][]int for j := 0; j < k; { item := make([]int, k) copy(item, temp[:k]) res = append(res, item) for j = 0; j < k && temp[j]+1 == temp[j+1]; j++ { temp[j] = j + 1 } temp[j]++ } return res } ``` ## [46. Permutations](https://leetcode.com/problems/permutations/) ### Description Given an array `nums` of distinct integers, return *all the possible permutations*. You can return the answer in **any order**. **Example 1:** ``` Input: nums = [1,2,3] Output: [[1,2,3],[1,3,2],[2,1,3],[2,3,1],[3,1,2],[3,2,1]] ``` **Example 2:** ``` Input: nums = [0,1] Output: [[0,1],[1,0]] ``` **Example 3:** ``` Input: nums = [1] Output: [[1]] ``` **Constraints:** * `1 <= nums.length <= 6` * `-10 <= nums[i] <= 10` * All the integers of `nums` are **unique**. ### Solution #### Approach #0 ```go func permute(nums []int) [][]int { var res [][]int var item []int m := make(map[int]struct{}) n := len(nums) var dfs func([]int, map[int]struct{}) dfs = func(item []int, m map[int]struct{}) { if len(item) == n { a := make([]int, n) copy(a, item) res = append(res, a) return } for i := 0; i < n; i++ { if _, ok := m[nums[i]]; ok { continue } item = append(item, nums[i]) m[nums[i]] = struct{}{} dfs(item, m) delete(m, nums[i]) item = item[:len(item)-1] } } dfs(item, m) return res } ``` #### Approach #1 ```go func permute(nums []int) [][]int { var res [][]int n := len(nums) var backtrack func(int) backtrack = func(first int) { if first == n { a := make([]int, n) copy(a, nums) res = append(res, a) return } for i := first; i < n; i++ { nums[first], nums[i] = nums[i], nums[first] backtrack(first + 1) nums[first], nums[i] = nums[i], nums[first] } } backtrack(0) return res } ``` ## [784. Letter Case Permutation](https://leetcode.com/problems/letter-case-permutation/) ### Description Given a string `s`, you can transform every letter individually to be lowercase or uppercase to create another string. Return *a list of all possible strings we could create*. Return the output in **any order**. **Example 1:** ``` Input: s = "a1b2" Output: ["a1b2","a1B2","A1b2","A1B2"] ``` **Example 2:** ``` Input: s = "3z4" Output: ["3z4","3Z4"] ``` **Constraints:** * `1 <= s.length <= 12` * `s` consists of lowercase English letters, uppercase English letters, and digits. ### Solution #### Approach #0: Too Slow ```go func letterCasePermutation(s string) []string { sList := []rune(s) n := len(s) var res []string var backtrace func(int) backtrace = func(start int) { if start == n { res = append(res, string(sList)) return } for i := start; i < len(sList); i++ { backtrace(i + 1) if !unicode.IsLetter(sList[i]) { continue } if unicode.IsLower(sList[i]) { sList[i] = unicode.ToUpper(sList[i]) backtrace(i + 1) sList[i] = unicode.ToLower(sList[i]) } if unicode.IsUpper(sList[i]) { sList[i] = unicode.ToLower(sList[i]) backtrace(i + 1) sList[i] = unicode.ToUpper(sList[i]) } } } backtrace(0) m := make(map[string]struct{}) var a []string for _, ss := range res { if _, ok := m[ss]; ok { continue } a = append(a, ss) m[ss] = struct{}{} } return a } ``` #### Approach #1: Much Better ```go func letterCasePermutation(s string) []string { res := []string{""} for _, ch := range s { n := len(res) if unicode.IsLetter(rune(ch)) { for i := 0; i < n; i++ { res = append(res, res[i]+string(unicode.ToLower(rune(ch)))) res[i] = res[i] + string(unicode.ToUpper(rune(ch))) } } else { for i := 0; i < n; i++ { res[i] = res[i] + string(ch) } } } return res } ``` --- # Agent Instructions This documentation is published with GitBook. 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