Stack: Push

Task: Push three values onto the stack

class Stack {    private Object[] a = new Object[N];    private int top = -1;        // -1 means empty     void push(Object v) { a[++top] = v; }    Object pop()        { return a[top--]; }    Object peek()       { return a[top]; }    int size()          { return top + 1; }}

What you are watching

Pushing adds a value at the top of the stack, and the top is the only place anything can be added. A, then B, then C: each one lands on the one before it, and each one becomes the value the stack will hand back first. A is still there the whole time and has been unreachable since B arrived, which is the rule the other three operations inherit.