最长递增序列2D矩阵递归

def longest_increasing_path(matrix):
    if not matrix:
        return 0

    def dfs(i, j):
        if mem[i][j]:
            return mem[i][j]
        val = matrix[i][j]
        mem[i][j] = 1 + max(
            dfs(i-1, j) if i and val > matrix[i-1][j] else 0,
            dfs(i+1, j) if i < m-1 and val > matrix[i+1][j] else 0,
            dfs(i, j-1) if j and val > matrix[i][j-1] else 0,
            dfs(i, j+1) if j < n-1 and val > matrix[i][j+1] else 0
        )
        return mem[i][j]

    m, n = len(matrix), len(matrix[0])
    mem = [[0]*n for _ in range(m)]
    ans = 0
    for i in range(m):
        for j in range(n):
            ans = max(ans, dfs(i, j))
    return ans