const commonChild = (s1: string, s2: string): number => {
    const m = s1.length;
    const n = s2.length;

    // @ts-ignore
    const matrix: number[][] = Array.from({length: m + 1}, () => Array(n + 1).fill(0));

    let longestChild = matrix[0][0];

    for (let i = 1; i <= m; i++) {
        for (let j = 1; j <= n; j++) {
            if (s1[i - 1] === s2[j - 1]) {
                matrix[i][j] = matrix[i - 1][j - 1] + 1;
            } else {
                matrix[i][j] = Math.max(matrix[i - 1][j], matrix[i][j - 1]);
            }

            if (matrix[i][j] > longestChild) {
                longestChild = matrix[i][j];
            }
        }
    }

    return longestChild;
}