# 2022-06-07

## [17. Letter Combinations of a Phone Number](https://leetcode.com/problems/letter-combinations-of-a-phone-number/)

### Description

Given a string containing digits from `2-9` inclusive, return all possible letter combinations that the number could represent. Return the answer in **any order**.

A mapping of digit to letters (just like on the telephone buttons) is given below. Note that 1 does not map to any letters.

![](https://img.content.cc/a/2022/06/07/13-29-24-602-b197f2cad4c616b374b89c2ed0444780-b2acd8.png)

**Example 1:**

```
Input: digits = "23"
Output: ["ad","ae","af","bd","be","bf","cd","ce","cf"]
```

**Example 2:**

```
Input: digits = ""
Output: []
```

**Example 3:**

```
Input: digits = "2"
Output: ["a","b","c"]
```

**Constraints:**

* `0 <= digits.length <= 4`
* `digits[i]` is a digit in the range `['2', '9']`.

### Solution

#### Approach #0

```go
var keyboard = map[byte]string {
    '2':"abc",
    '3':"def",
    '4':"ghi",
    '5':"jkl",
    '6':"mno",
    '7':"pqrs",
    '8':"tuv",
    '9':"wxyz",
}
func letterCombinations(digits string) (ans []string) {
    n:=len(digits)
    if n==0 {
        return
    }
    var b func(cur int, str string)
    b=func(cur int, str string) {
        if cur==n {
            ans = append(ans, str)
            return
        }
        for _, ch := range keyboard[digits[cur]] {
            b(cur+1, str+string(ch))
        }
    }
    b(0,"")
    return
}
```

## [22. Generate Parentheses](https://leetcode.com/problems/generate-parentheses/)

### Description

Given `n` pairs of parentheses, write a function to *generate all combinations of well-formed parentheses*.

**Example 1:**

```
Input: n = 3
Output: ["((()))","(()())","(())()","()(())","()()()"]
```

**Example 2:**

```
Input: n = 1
Output: ["()"]
```

**Constraints:**

* `1 <= n <= 8`

### Solution

#### Approach #0

```go
func generateParenthesis(n int) (ans []string) {
    var tmp []byte
    var b func(left, right int)
    b = func(left, right int) {
        if len(tmp) == n*2 {
            ans = append(ans, string(tmp))
            return
        }
        if left < n {
            tmp = append(tmp, '(')
            b(left+1, right)
            tmp = tmp[:len(tmp)-1]
        }
        if right < left {
            tmp = append(tmp, ')')
            b(left, right+1)
            tmp = tmp[:len(tmp)-1]
        }
    }
    b(0, 0)
    return
}
```

## [79. Word Search](https://leetcode.com/problems/word-search/)

### Description

Given an `m x n` grid of characters `board` and a string `word`, return `true` *if* `word` *exists in the grid*.

The word can be constructed from letters of sequentially adjacent cells, where adjacent cells are horizontally or vertically neighboring. The same letter cell may not be used more than once.

**Example 1:**

![](https://img.content.cc/a/2022/06/07/14-13-19-918-ab76dd0b1a371e0c27351ea17d1fdc75-b5aa49.png)

```
Input: board = [["A","B","C","E"],["S","F","C","S"],["A","D","E","E"]], word = "ABCCED"
Output: true
```

**Example 2:**

![](https://img.content.cc/a/2022/06/07/14-13-35-465-1cfbb079ff914b5b58e7b49de59b55cf-046f8c.png)

```
Input: board = [["A","B","C","E"],["S","F","C","S"],["A","D","E","E"]], word = "SEE"
Output: true
```

**Example 3:**

![](https://img.content.cc/a/2022/06/07/14-13-47-964-34311952a774f4e210dd93c176874bfc-edccd0.png)

```
Input: board = [["A","B","C","E"],["S","F","C","S"],["A","D","E","E"]], word = "ABCB"
Output: false
```

**Constraints:**

* `m == board.length`
* `n = board[i].length`
* `1 <= m, n <= 6`
* `1 <= word.length <= 15`
* `board` and `word` consists of only lowercase and uppercase English letters.

**Follow up:** Could you use search pruning to make your solution faster with a larger `board`?

### Solution

#### Approach #0

```go
var (
    dx = []int{0, 1, 0, -1}
    dy = []int{1, 0, -1, 0}
)

func exist(board [][]byte, word string) bool {
    m, n := len(board), len(board[0])
    vis := make([][]bool, m)
    for i := range vis {
        vis[i] = make([]bool, n)
    }
    var b func(x, y, index int) bool
    b = func(x, y, index int) bool {
        if board[x][y] != word[index] {
            return false
        }
        if index == len(word)-1 {
            return true
        }
        vis[x][y] = true
        defer func() { vis[x][y] = false }()
        for i := 0; i < 4; i++ {
            xx, yy := x+dx[i], y+dy[i]
            if xx >= 0 && xx < m && yy >= 0 && yy < n && !vis[xx][yy] {
                if b(xx, yy, index+1) {
                    return true
                }
            }
        }
        return false
    }
    for i, row := range board {
        for j := range row {
            if b(i, j, 0) {
                return true
            }
        }
    }
    return false
}
```


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