int commonChild(string s1, string s2)
{
    int m = s1.length();
    int n = s2.length();
    
    vector<int> curr(n+1,0), prev(n+1,0);
    
    for(int i=1;i<=m;++i)
    {
        for(int j=1;j<=n;++j)
        {
            if(s1[i-1] == s2[j-1])
            curr[j] = 1 + prev[j-1];
            else
             curr[j] = max(curr[j-1],prev[j]);
        }
        prev = curr;
    }
    return curr[n];
}

def commonChild(s1, s2):
    arr = [[0 for _ in range(len(s1)+1)] for _ in range(len(s2)+1) ]
    for i in range(1,len(s1)+1,1):
        for j in range(1,len(s2)+1,1):
            arr[i][j]=max(
                ((arr[i-1][j-1] +1) if s1[i-1]==s2[j-1] else arr[i-1][j-1]),
                arr[i-1][j],
                arr[i][j-1]
            )
    return arr[-1][-1] 

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;
}

def commonChild(s1, s2):
    # Write your code here
    prev=[-1 for i in range(len(s2)+1)]
    for i in range(len(s1),-1,-1):
        curr=[-1 for i in range(len(s2)+1)]
        for j in range(len(s2),-1,-1):
            if i==len(s1) or j==len(s2):
                curr[j] = 0
            else:
                if s1[i]==s2[j]:
                    curr[j] = 1+prev[j+1]
                else:
                    curr[j] = max(prev[j],curr[j+1])
        prev=curr
    return curr[0]