Multi-pass shapes

Two multi-pass shapes complete the toolkit:

Count-then-act. "Move all zeros to the end, preserving order of the rest." Pass 1 counts zeros; pass 2 writes non-zeros forward and fills the tail with zeros:

int next = 0;
for (int i = 0; i < arr.length; i++) {
    if (arr[i] != 0) { arr[next] = arr[i]; next++; }
}
for (int i = next; i < arr.length; i++) { arr[i] = 0; }

Trace {0, 3, 0, 1}: next walks 0, 1, 1, 2 — after pass one, {3, 1, 0, 1} (the tail is garbage); pass two zeroes it: {3, 1, 0, 0}. Order preserved, no extra array.

Condition-based dedup — "count elements NOT already seen". With no extra structures allowed, compare each element only to the previous distinct run, or accept O(n²) pairwise comparison:

int distinct = 0;
for (int i = 0; i < arr.length; i++) {
    boolean seen = false;
    for (int j = 0; j < i; j++) {
        if (arr[j] == arr[i]) { seen = true; break; }
    }
    if (!seen) { distinct++; }
}

{5, 3, 5, 1} → distinct = 3. The inner loop only looks backward (j < i), so the first occurrence claims each value. On the exam this pairwise form is acceptable — clarity beats cleverness, and the arrays are short.