問題

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typescript

typescript

function longestPalindrome(s: string): string {
  if (!s || s.length <= 1) return s
  let longest = s.substring(0, 1)

  for (let i = 0; i < s.length; i++) {
    let left = i
    let right = i
  
    while (left >= 0 && right < s.length && s[left] === s[right]) {
      left -= 1
      right += 1
    }
    const oddPalindrome = s.slice(left + 1, right)

    left = i
    right = i + 1
  
    while (left >= 0 && right < s.length && s[left] === s[right]) {
      left -= 1
      right += 1
    }
    const evenPalindrome = s.slice(left + 1, right)

    if (Math.max(oddPalindrome.length, evenPalindrome.length) > longest.length) {
      longest = oddPalindrome.length > evenPalindrome.length ? oddPalindrome : evenPalindrome
    }
  }
  return longest
}

• 回文には、奇数の長さになるものと偶数の長さになるものがある
• 最初の文字に対して、左に右に1文字ずつ同じかどうか確認しながら、増やしていく