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POUse an array to implement three Stack
 

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    <p>I saw the following Java code used to implement three stack on a single array.</p> <pre><code>1 int stackSize = 300; 2 int indexUsed = 0; 3 int[] stackPointer = {-1,-1,-1}; 4 StackNode[] buffer = new StackNode[stackSize * 3]; 5 void push(int stackNum, int value) { 6 int lastIndex = stackPointer[stackNum]; 7 stackPointer[stackNum] = indexUsed; 8 indexUsed++; 9 buffer[stackPointer[stackNum]]=new StackNode(lastIndex,value); 10 } 11 int pop(int stackNum) { 12 int value = buffer[stackPointer[stackNum]].value; 13 int lastIndex = stackPointer[stackNum]; 14 stackPointer[stackNum] = buffer[stackPointer[stackNum]].previous; 15 buffer[lastIndex] = null; 16 indexUsed--; 17 return value; 18 } 19 int peek(int stack) { return buffer[stackPointer[stack]].value; } 20 boolean isEmpty(int stackNum) { return stackPointer[stackNum] == -1; } 21 22 class StackNode { 23 public int previous; 24 public int value; 25 public StackNode(int p, int v){ 26 value = v; 27 previous = p; 28 } 29 } </code></pre> <p>My question1 is that Line 4 has allocated memory for the variable buffer. Why in Line 9, we still need to allocate new StackNode.</p> <p>My question2 is: Can the function pop help recollect the used memory?</p> <p>For example,</p> <pre><code>Stack1_E1 =&gt; Stack1_E2 =&gt; Stack2_E1 =&gt; Stack2_E2 =&gt; Stack3_E1 </code></pre> <p>When we call pop(0) // pop the Stack1 Based on my understanding, the free space used by Stack1_E2 will not be reused next time when we call push.</p> <p>Is the function pop designed correctly? Thank you</p> <p>Note: This question has been modified and includes the pop function.</p>
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    1. COPlease DO NOT remove the line number from my question. == Thank you
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