Two Pointers (Same Direction)

When to use

• Remove duplicates
• Partitioning
• In-place compression

Invariant

Elements [0 … slow] are already in final valid form.

Java

static void removeDuplicates(int[] a) {
    int slow = 0;
    for (int fast = 1; fast < a.length; fast++) {
        if (a[fast] != a[slow])
            a[++slow] = a[fast];
    }
}