Question
Implement a LIFO stack that has a push(), pop(), and max() function, where max() returns the maximum value in the stack. All of these functions should run in O(1) time.
eg.
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push(1) max() = 1 push(2) max() = 2 push(1) max() = 2 pop() = 1 max() = 2 pop() = 2 max() = 1 |
Once you think that you’ve solved the problem, click below to see the solution.
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Solution
How was that problem? You can check out the solution in the video below.
Here is the source code for the solution shown in the video:
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public class MaxStack { private class Node { private int value; private Node next; private Node oldMax; } private Node stack; private Node max; public MaxStack() {} public void push(int x) { Node n = new Node(); n.value = x; if (stack == null) { stack = n; } else { n.next = stack; stack = n; } if (max == null || n.value > max.value) { n.oldMax = max; max = n; } } public int pop() { if (stack == null) throw new NullPointerException; Node n = stack; stack = n.next; max = n.oldMax; return n.value; } public int max() { if (max == null) throw new NullPointerException; return max.value; } } |
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