Pattern
Pattern
Pattern
Use a matrix or two-dimensional state when decisions depend on two indexes.
Define dp[row][col] or dp[left][right] and fill in dependency order.
dp = [[0] * cols for _ in range(rows)]
for row in range(rows):
dp[row][0] = 1
for col in range(cols):
dp[0][col] = 1
for row in range(1, rows):
for col in range(1, cols):
dp[row][col] = dp[row - 1][col] + dp[row][col - 1]
answer = dp[-1][-1]