# 2022-06-11

## [91. Decode Ways](https://leetcode.com/problems/decode-ways/)

### Description

A message containing letters from `A-Z` can be **encoded** into numbers using the following mapping:

```
'A' -> "1"
'B' -> "2"
...
'Z' -> "26"
```

To **decode** an encoded message, all the digits must be grouped then mapped back into letters using the reverse of the mapping above (there may be multiple ways). For example, `"11106"` can be mapped into:

* `"AAJF"` with the grouping `(1 1 10 6)`
* `"KJF"` with the grouping `(11 10 6)`

Note that the grouping `(1 11 06)` is invalid because `"06"` cannot be mapped into `'F'` since `"6"` is different from `"06"`.

Given a string `s` containing only digits, return *the **number** of ways to **decode** it*.

The test cases are generated so that the answer fits in a **32-bit** integer.

**Example 1:**

```
Input: s = "12"
Output: 2
Explanation: "12" could be decoded as "AB" (1 2) or "L" (12).
```

**Example 2:**

```
Input: s = "226"
Output: 3
Explanation: "226" could be decoded as "BZ" (2 26), "VF" (22 6), or "BBF" (2 2 6).
```

**Example 3:**

```
Input: s = "06"
Output: 0
Explanation: "06" cannot be mapped to "F" because of the leading zero ("6" is different from "06").
```

**Constraints:**

* `1 <= s.length <= 100`
* `s` contains only digits and may contain leading zero(s).

### Solution

#### Approach #0

```go
func numDecodings(s string) int {
    n := len(s)
    a, b, c := 0, 1, 0
    for i := 1; i <= n; i++ {
        c = 0
        if s[i-1] != '0' {
            c = b
        }
        if i > 1 && s[i-2] != '0' && (s[i-2]-'0')*10+(s[i-1]-'0') <= 26 {
            c += a
        }
        a, b = b, c
    }
    return c
}
```

## [139. Word Break](https://leetcode.com/problems/word-break/)

### Description

Given a string `s` and a dictionary of strings `wordDict`, return `true` if `s` can be segmented into a space-separated sequence of one or more dictionary words.

**Note** that the same word in the dictionary may be reused multiple times in the segmentation.

**Example 1:**

```
Input: s = "leetcode", wordDict = ["leet","code"]
Output: true
Explanation: Return true because "leetcode" can be segmented as "leet code".
```

**Example 2:**

```
Input: s = "applepenapple", wordDict = ["apple","pen"]
Output: true
Explanation: Return true because "applepenapple" can be segmented as "apple pen apple".
Note that you are allowed to reuse a dictionary word.
```

**Example 3:**

```
Input: s = "catsandog", wordDict = ["cats","dog","sand","and","cat"]
Output: false
```

**Constraints:**

* `1 <= s.length <= 300`
* `1 <= wordDict.length <= 1000`
* `1 <= wordDict[i].length <= 20`
* `s` and `wordDict[i]` consist of only lowercase English letters.
* All the strings of `wordDict` are **unique**.

### Solution

#### Approach #0

```go
func wordBreak(s string, wordDict []string) bool {
    m := make(map[string]bool)
    for _, word := range wordDict {
        m[word] = true
    }
    dp := make([]bool, len(s)+1)
    dp[0] = true
    for i := 1; i <= len(s); i++ {
        for j := 0; j < i; j++ {
            if dp[j] && m[s[j:i]] {
                dp[i] = true
                break
            }
        }
    }
    return dp[len(s)]
}
```


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