# 2022-06-10

## [5. Longest Palindromic Substring](https://leetcode.com/problems/longest-palindromic-substring/)

### Description

Given a string `s`, return *the longest palindromic substring* in `s`.

**Example 1:**

```
Input: s = "babad"
Output: "bab"
Explanation: "aba" is also a valid answer.
```

**Example 2:**

```
Input: s = "cbbd"
Output: "bb"
```

**Constraints:**

* `1 <= s.length <= 1000`
* `s` consist of only digits and English letters.

### Solution

#### Approach #0

```go
func longestPalindrome(s string) string {
    n := len(s)
    if n < 2 {
        return s
    }
    dp := make([][]bool, n)
    for i := 0; i < n; i++ {
        dp[i] = make([]bool, n)
        dp[i][i] = true
    }

    maxLen := 1
    start := 0

    for c := 2; c <= n; c++ {
        for i := 0; i < n; i++ {
            j := c + i - 1
            if j >= n {
                break
            }

            if s[i] != s[j] {
                dp[i][j] = false
            } else {
                if j-i < 3 {
                    dp[i][j] = true
                } else {
                    dp[i][j] = dp[i+1][j-1]
                }
            }

            if dp[i][j] && j-i+1 > maxLen {
                maxLen = j - i + 1
                start = i
            }
        }
    }
    return s[start : start+maxLen]
}
```

## [413. Arithmetic Slices](https://leetcode.com/problems/arithmetic-slices/)

### Description

An integer array is called arithmetic if it consists of **at least three elements** and if the difference between any two consecutive elements is the same.

* For example, `[1,3,5,7,9]`, `[7,7,7,7]`, and `[3,-1,-5,-9]` are arithmetic sequences.

Given an integer array `nums`, return *the number of arithmetic **subarrays** of* `nums`.

A **subarray** is a contiguous subsequence of the array.

**Example 1:**

```
Input: nums = [1,2,3,4]
Output: 3
Explanation: We have 3 arithmetic slices in nums: [1, 2, 3], [2, 3, 4] and [1,2,3,4] itself.
```

**Example 2:**

```
Input: nums = [1]
Output: 0
```

**Constraints:**

* `1 <= nums.length <= 5000`
* `-1000 <= nums[i] <= 1000`

### Solution

#### Approach #0: Too much memory used

```go
func numberOfArithmeticSlices(nums []int) (ans int) {
    n := len(nums)
    if n < 3 {
        return 0
    }
    v := make([]int, n-1)
    for i := 0; i < n-1; i++ {
        v[i] = nums[i+1] - nums[i]
    }

    for c := 3; c <= n; c++ {
        for i := 0; i < n; i++ {
            if i+c > n {
                break
            }
            p := true
            for j := i; j < i+c-2; j++ {
                p = p && v[j] == v[j+1]
                if !p {
                    break
                }
            }
            if p {
                ans++
            }
        }
    }
    return
}
```

#### Approach #1

```go
func numberOfArithmeticSlices(nums []int) (ans int) {
    n := len(nums)
    if n == 1 {
        return
    }
    d, t := nums[1]-nums[0], 0
    for i := 2; i < n; i++ {
        if nums[i]-nums[i-1] == d {
            t++
        } else {
            d, t = nums[i]-nums[i-1], 0
        }
        ans += t
    }
    return
}
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://a.b.cr/dictionary/algorithm/diary/2022/06/2022-06-10.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.