Three in One: Describe how you could use a single array to implement three stacks.
Hints: #2, #12, #38, #58
解答
如果数组的长度固定，那么可以把数组分割成3份，每份各司其职。不过这个似乎太简单了。
如果说不固定3个数组的长度，即抢占式的。那么似乎每个stack都记录自己的element的下标。
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public class Stack {
private Object[] array;
private int[] elementIndex;
private int size;
public Stack(Object[] array) {
this.array = array;
this.elementIndex = new int[array.length];
for (int i = 0; i < elementIndex.length; i++) {
elementIndex[i] = -1;
}
}
public Object peek() {
if (isEmpty()) {
return null;
}
int slot = elementIndex[size - 1];
return array[slot];
}
public boolean push(Object value) {
int slot = -1;
for (int i = 0; i < array.length; i++) {
if (array[i] == null) {
slot = i;
break;
}
}
if (slot == -1) {
return false;
}
array[slot] = value;
elementIndex[size] = slot;
size++;
return true;
}
public Object pop() {
if (isEmpty()) {
return null;
}
int slot = elementIndex[size - 1];
Object data = array[slot];
array[slot] = null;
elementIndex[size - 1] = -1;
size--;
return data;
}
public boolean isEmpty() {
return size == 0;
}
}
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