题解 | #密码截取#

const rl = require("readline").createInterface({ input: process.stdin });
var iter = rl[Symbol.asyncIterator]();
const readline = async () => (await iter.next()).value;

void (async function () {
  const s = (await readline()).split("");
  const n = s.length;
  
  // 是一个倒三角，这样节省空间
  const dp = new Array(n + 1).fill(0).map((_, index) => Array(n - index + 1).fill(0));

  // dp[i][j] = 1 表示从i开始，长度为j是否为回文字符串

  // 单字符串都是回文字符串
  for (let i = 0; i < n; i++) {
    dp[i][1] = 1;
  }

  let width = 1; // 单字符都是回文字符
  let max = 1;
  while (width < n) {
    width++; // 当前遍历的字符长度
    for (let i = 0; i < n; i++) {
      let j = i + width - 1;
      if(j >= n) {
        continue;
      }
      // console.log(s[i], s[j], i, j, width)
      if (s[i] === s[j]) {
        // console.log(s[i], s[j], i, j, dp[i+1][width - 2])
        if(width === 2) {
          dp[i][2] = 1
          max = 2
        } else if(dp[i+1][width - 2]) {
          dp[i][width] = 1
          max = Math.max(max, width)
        }
      }
    }
  }

  // dp.forEach(n => console.log(n.join(' ')))
  console.log(max);

  // 求最长的回文字符串
})();