Distinct Subsequences

def numDistinct(s, t):
    cache = {}
    
    def dfs(i, j):
        if j == len(t):
            return 1
        if i == len(s):
            return 0
        if (i, j) in cache:
            return cache[(i, j)]
            
        if s[i] == t[j]:
            cache[(i, j)] = dfs(i + 1, j + 1) + dfs(i + 1, j)
        else:
            cache[(i, j)] = dfs(i + 1, j)
            
        return cache[(i, j)]
        
    return dfs(0, 0)