array_build_stack

Describe how you could use a single array to implement three stacks

/**Solution: One idea is to seperate the array into three equal chunk. but if one of them still have space left while another
/**is already full, space will be wasted
/**So in the following method, any stack can grow as long as there is any free space in the array 
/** but there is a bug for the provided answer, during pop() and collect space back
Class Stack{

    int stackSize = 300; 
    int indexUsed = 0; 
    int[] stackPointer = {-1,-1,-1}; 
    StackNode[] buffer = new StackNode[stackSize * 3]; 
    void push(int stackNum, int value) { 
        int lastIndex = stackPointer[stackNum]; 
        stackPointer[stackNum] = indexUsed; 
        indexUsed++; 
        buffer[stackPointer[stackNum]]=new StackNode(lastIndex,value); 
    } 
    /** for line 24, even we set buffer[lastIndex] = null, it cannot delete this position and shift all elements right to it left
    /**so it still cannot be used again, and as indexUsed decrese pointing to the allocated position in array
    /**it will rewrite the data in next push() operation
    int pop(int stackNum) { 
        int value = buffer[stackPointer[stackNum]].value; 
        int lastIndex = stackPointer[stackNum]; 
        stackPointer[stackNum] = buffer[stackPointer[stackNum]].previous; 
        buffer[lastIndex] = null; 
        indexUsed--; 
        return value; 
    } 
    
    int peek(int stack) { 
    return buffer[stackPointer[stack]].value; 
    } 
    
    boolean isEmpty(int stackNum) { 
    return stackPointer[stackNum] == -1; 
    } 
}
class StackNode { 
    public int previous; 
    public int value; 
    public StackNode(int p, int v){
       value = v; 
       previous = p; 
       } 
    }