Dimensions Quick Reference
| What You Need | Code |
|---|---|
| Number of rows | grid.length |
| Number of columns | grid[0].length |
| Access element at row r, col c | grid[r][c] |
| Last row index | grid.length - 1 |
| Last col index | grid[0].length - 1 |
Row-Major Traversal (Default)
for (int r = 0; r < grid.length; r++)
{
for (int c = 0; c < grid[0].length; c++)
{
// process grid[r][c]
}
}
Use for: visiting every cell, accumulating a total, counting matches, printing all values. This is the default traversal for almost every problem.
Column-Major Traversal
for (int c = 0; c < grid[0].length; c++)
{
for (int r = 0; r < grid.length; r++)
{
// process grid[r][c]
}
}
Use for: column sums, visiting cells top-to-bottom within each column, or when the problem specifies column-first order.
Row Sum Pattern
for (int r = 0; r < grid.length; r++)
{
int rowSum = 0;
for (int c = 0; c < grid[0].length; c++)
{
rowSum += grid[r][c];
}
System.out.println("Row " + r + " sum: " + rowSum);
}
Column Sum Pattern
for (int c = 0; c < grid[0].length; c++)
{
int colSum = 0;
for (int r = 0; r < grid.length; r++)
{
colSum += grid[r][c];
}
System.out.println("Col " + c + " sum: " + colSum);
}
Find Max in Entire Grid
int max = grid[0][0];
for (int r = 0; r < grid.length; r++)
{
for (int c = 0; c < grid[0].length; c++)
{
if (grid[r][c] > max)
{
max = grid[r][c];
}
}
}
Count Matches
int count = 0;
for (int r = 0; r < grid.length; r++)
{
for (int c = 0; c < grid[0].length; c++)
{
if (grid[r][c] == target)
{
count++;
}
}
}
Main Diagonal (Square Grid Only)
// Only works when grid.length == grid[0].length
for (int i = 0; i < grid.length; i++)
{
// Main diagonal: grid[i][i]
// Anti-diagonal: grid[i][grid.length - 1 - i]
}
Check Neighbors (With Bounds)
// Check all 4 neighbors of grid[r][c]
if (r > 0) { /* grid[r-1][c] = above */ }
if (r < grid.length - 1) { /* grid[r+1][c] = below */ }
if (c > 0) { /* grid[r][c-1] = left */ }
if (c < grid[0].length - 1) { /* grid[r][c+1] = right */ }
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