Row and column aggregation

Aggregation over a grid is the same accumulator pattern from 1D arrays, applied per-row, per-column, or over everything:

// row sums: one answer per row
public static int[] rowSums(int[][] g) {
    int[] sums = new int[g.length];
    for (int r = 0; r < g.length; r++) {
        int s = 0;                       // reset PER ROW — inside loop 1
        for (int c = 0; c < g[0].length; c++) {
            s += g[r][c];
        }
        sums[r] = s;
    }
    return sums;
}

Two placement bugs that cause half the wrong answers:

• Accumulator declared outside the row loop: sums accumulate down the whole grid (running total instead of per-row totals).
• Reset placed inside the column loop: every accumulator dies after one column.

Column traversal just swaps which loop is outside:

for (int c = 0; c < g[0].length; c++) {      // columns outside
    for (int r = 0; r < g.length; r++) {
        // g[r][c] walks DOWN column c
    }
}

Both loops stay; only their bounds and index order swap.