input: Write a function to find the number of unique paths from a given source to a destination in a grid of size m x n. The function should return the number of unique paths from the top-left corner to the bottom-right corner. You can only move either down or right at any point in time. If the grid size is 0, return 0.
  def num_unique_paths(m, n):
      if m == 0 or n == 0:
          return 0
      dp = [[0 for x in range(m)] for y in range(n)] 
    
      for i in range(n): 
          dp[i][0] = 1
    
      for j in range(m): 
          dp[0][j] = 1
    
      for i in range(1, n): 
          for j in range(1, m): 
              dp[i][j] = dp[i-1][j] + dp[i][j-1]
    
      return dp[n-1][m-1]
  assert num_unique_paths(3, 2) == 3
  assert num_unique_paths(3, 3) == 6
  assert num_unique_paths(0, 5) == 0
  assert num_unique_paths(4, 1) == 1
  assert num_unique_paths(1, 1) == 1
output: def num_unique_paths(m, n):
      if m == 0 or n == 0:
          return 0
      dp = [[0 for x in range(m)] for y in range(n)] 
    
      for i in range(n): 
          dp[i][0] = 1
    
      for j in range(m): 
          dp[0][j] = 1
    
      for i in range(1, n): 
          for j in range(1, m): 
              dp[i][j] = dp[i-1][j] + dp[i][j-1]
    
      return dp[n-1][m-1]