## Problem
https://leetcode.com/problems/valid-palindrome/

<br>

:::spoiler **Optimal Space & Time Complexity**

:::

<br>

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## Solutions

:::spoiler 東

:::

<br>

:::spoiler Hao

// Time complexity: O(n) where n === s.length
// Space complexity: O(1)

const codes = {
    '0': 48,
    '9': 57,
    'A': 65,
    'Z': 90,
    'a': 97,
    'z': 122,
};

const upperLowerCodeDiff = codes['A'] - codes['a'];

const isNumber = (code) => code >= codes['0'] && code <= codes['9'];

const isUpperAlphabet = (code) => code >= codes['A'] && code <= codes['Z'];

const isLowerAlphabet = (code) => code >= codes['a'] && code <= codes['z'];

const isAlphanumeric = (code) => isNumber(code) || isUpperAlphabet(code) || isLowerAlphabet(code);

const isEqualAlphanumeric = (codeLeft, codeRight) => {
    if (isNumber(codeLeft) && isNumber(codeRight)) return codeLeft === codeRight;
    else if (!isNumber(codeLeft) && !isNumber(codeRight)) {
        const lowerCodeLeft = isUpperAlphabet(codeLeft) ? codeLeft - upperLowerCodeDiff : codeLeft;
        const lowerCodeRight = isUpperAlphabet(codeRight) ? codeRight - upperLowerCodeDiff : codeRight;
        
        return lowerCodeLeft === lowerCodeRight;
    } else return false
};


var isPalindrome = function(s) {
    let res = s.length > 0;

    let left = 0;
    let right = s.length - 1;

    while (res && left <= right) {
        const codeLeft = s.charCodeAt(left);
        const codeRight = s.charCodeAt(right);

        if (!isAlphanumeric(codeLeft)) {
            left += 1;
            continue;
        }

        if (!isAlphanumeric(codeRight)) {
            right -= 1;
            continue;
        }

        res = isEqualAlphanumeric(codeLeft, codeRight);

        left += 1;
        right -= 1;
    }

    return res;
};

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<br>

:::spoiler YC

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<br>

:::spoiler SOL

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## Supplement / Discussion

### 東

### Hao

- string ⇒ array ⇒ 線性結構 ⇒ palindrome ⇒ two pointer ⇒ 左右指針
- converting all uppercase letters into lowercase letters
- uppercase = lowercase
- removing all non-alphanumeric characters, characters other than letters and numbers
- How to differentiate? `String.prototype.charCodeAt()`

| chars | codes |
| --- | --- |
| 0-9 | 48-57 |
| A-Z | 65-90 |
| a-z | 97-122 |

- Could replace self-implemented func `isEqualAlphanumeric()` with native `string.prototype.toLowerCase()`

### YC

### SOL