Grid Tips & CCC Reference

The Checklist

Before writing any grid function:

• Know your grid size: rows = len(grid), cols = len(grid[0])
• Use grid[row][col] — row first, then col
• Validate before accessing: 0 <= r < rows and 0 <= c < cols
• Track visited cells to avoid infinite recursion

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Boundary Check — Never Skip This

# Always check before accessing a cell
if 0 <= r < rows and 0 <= c < cols:
    val = grid[r][c]  # safe

Missing this causes IndexError or silent wrong answers.

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Visited Set vs Visited Grid

# Option 1: set of tuples (easy)
visited = set()
visited.add((row, col))
if (row, col) in visited: ...

# Option 2: boolean grid (faster for large grids)
visited = [[False] * cols for _ in range(rows)]
visited[row][col] = True
if visited[row][col]: ...

Both work. Sets are simpler to write in a contest.

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DFS vs BFS — When to Use Each

Problem	Use
Does a path exist?	DFS
Count all regions / islands	DFS
Flood fill	DFS
Shortest path length	BFS
Minimum steps to reach target	BFS

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Common Mistakes

Wrong index order:

# WRONG — trying to use (x, y) like math coordinates
grid[col][row]

# RIGHT — grid is indexed row first
grid[row][col]

Forgetting to mark visited before recursing:

# WRONG — infinite loop if two neighbors point to each other
def dfs(r, c):
    dfs(r+1, c)  # visits (r+1,c), which calls dfs(r,c) again → infinite!

# RIGHT — mark before recursing
def dfs(r, c):
    visited.add((r, c))
    dfs(r+1, c)

Not initializing cols correctly for non-square grids:

# WRONG for non-square grids
cols = len(grid)  # this is rows!

# RIGHT
rows = len(grid)
cols = len(grid[0])

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CCC Q5 Tips

• Draw it on paper — trace a 3×3 example by hand before coding
• Start small — get the 2×2 case working first
• Separate concerns — write a helper is_valid(r, c) if you need bounds checks repeatedly
• BFS guarantees shortest — if the problem asks for minimum distance, reach for BFS first