Longest Palindrome After Substring Concatenation II
<p>You are given two strings, <code>s</code> and <code>t</code>.</p>

<p>You can create a new string by selecting a <span data-keyword="substring">substring</span> from <code>s</code> (possibly empty) and a substring from <code>t</code> (possibly empty), then concatenating them <strong>in order</strong>.</p>

<p>Return the length of the <strong>longest</strong> <span data-keyword="palindrome-string">palindrome</span> that can be formed this way.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<div class="example-block">
<p><strong>Input:</strong> <span class="example-io">s = &quot;a&quot;, t = &quot;a&quot;</span></p>

<p><strong>Output:</strong> <span class="example-io">2</span></p>

<p><strong>Explanation:</strong></p>

<p>Concatenating <code>&quot;a&quot;</code> from <code>s</code> and <code>&quot;a&quot;</code> from <code>t</code> results in <code>&quot;aa&quot;</code>, which is a palindrome of length 2.</p>
</div>

<p><strong class="example">Example 2:</strong></p>

<div class="example-block">
<p><strong>Input:</strong> <span class="example-io">s = &quot;abc&quot;, t = &quot;def&quot;</span></p>

<p><strong>Output:</strong> <span class="example-io">1</span></p>

<p><strong>Explanation:</strong></p>

<p>Since all characters are different, the longest palindrome is any single character, so the answer is 1.</p>
</div>

<p><strong class="example">Example 3:</strong></p>

<div class="example-block">
<p><strong>Input:</strong> <span class="example-io">s = &quot;b&quot;, t = &quot;aaaa&quot;</span></p>

<p><strong>Output:</strong> <span class="example-io">4</span></p>

<p><strong>Explanation:</strong></p>

<p>Selecting &quot;<code>aaaa</code>&quot; from <code>t</code> is the longest palindrome, so the answer is 4.</p>
</div>

<p><strong class="example">Example 4:</strong></p>

<div class="example-block">
<p><strong>Input:</strong> <span class="example-io">s = &quot;abcde&quot;, t = &quot;ecdba&quot;</span></p>

<p><strong>Output:</strong> <span class="example-io">5</span></p>

<p><strong>Explanation:</strong></p>

<p>Concatenating <code>&quot;abc&quot;</code> from <code>s</code> and <code>&quot;ba&quot;</code> from <code>t</code> results in <code>&quot;abcba&quot;</code>, which is a palindrome of length 5.</p>
</div>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= s.length, t.length &lt;= 1000</code></li>
	<li><code>s</code> and <code>t</code> consist of lowercase English letters.</li>
</ul>
Two Pointers; String; Dynamic Programming
function longestPalindrome(s: string, t: string): number {
    function expand(s: string, g: number[], l: number, r: number): void {
        while (l >= 0 && r < s.length && s[l] === s[r]) {
            g[l] = Math.max(g[l], r - l + 1);
            l--;
            r++;
        }
    }

    function calc(s: string): number[] {
        const n = s.length;
        const g: number[] = Array(n).fill(0);
        for (let i = 0; i < n; i++) {
            expand(s, g, i, i);
            expand(s, g, i, i + 1);
        }
        return g;
    }

    const m = s.length,
        n = t.length;
    t = t.split('').reverse().join('');
    const g1 = calc(s);
    const g2 = calc(t);
    let ans = Math.max(...g1, ...g2);

    const f: number[][] = Array.from({ length: m + 1 }, () => Array(n + 1).fill(0));

    for (let i = 1; i <= m; i++) {
        for (let j = 1; j <= n; j++) {
            if (s[i - 1] === t[j - 1]) {
                f[i][j] = f[i - 1][j - 1] + 1;
                ans = Math.max(ans, f[i][j] * 2 + (i >= m ? 0 : g1[i]));
                ans = Math.max(ans, f[i][j] * 2 + (j >= n ? 0 : g2[j]));
            }
        }
    }

    return ans;
}
