Min Stack: the idea to remember.

The core insight

Store each pushed value together with the minimum at that depth.

Why it works

The top pair remembers the minimum of exactly the surviving stack prefix, even after pops.

The mistake to avoid

A single global minimum cannot recover the previous minimum after a pop.

Commented Java solution

Use java.util.* imports and a surrounding solution class. Tree snippets assume the standard TreeNode definition; linked lists use ListNode and Clone Graph uses a Node with val and neighbors; Copy Random List uses Node with val, next and random. Read the input assumptions above.

class MinStack {
    private final Deque<int[]> stack = new ArrayDeque<>();
    public void push(int value) {
        int min = stack.isEmpty() ? value : Math.min(value, stack.peek()[1]);
        stack.push(new int[]{value, min});
    }
    public void pop() { stack.pop(); }
    public int top() { return stack.peek()[0]; }
    public int getMin() { return stack.peek()[1]; }
}

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