Longest Increasing Path in a Matrix

def longestIncreasingPath(matrix):
    ROWS, COLS = len(matrix), len(matrix[0])
    dp = {} # (r, c) -> LIP
    
    def dfs(r, c, prevVal):
        if (r < 0 or r == ROWS or 
            c < 0 or c == COLS or 
            matrix[r][c] <= prevVal):
            return 0
        if (r, c) in dp:
            return dp[(r, c)]
            
        res = 1
        res = max(res, 1 + dfs(r + 1, c, matrix[r][c]))
        res = max(res, 1 + dfs(r - 1, c, matrix[r][c]))
        res = max(res, 1 + dfs(r, c + 1, matrix[r][c]))
        res = max(res, 1 + dfs(r, c - 1, matrix[r][c]))
        
        dp[(r, c)] = res
        return res
        
    for r in range(ROWS):
        for c in range(COLS):
            dfs(r, c, -1)
            
    return max(dp.values())