Cracking the Coding Interview, 4 Edition - 150 Programming Interview Questions and Solutions

Solutions to Chapter 3 | Stacks and Queues
3.1 Describe how you could use a single array to implement three stacks.
pg 52
SOLUTION
Approach 1:
Divide the array in three equal parts and allow the individual stack to grow in that limited
space (note: “[“ means inclusive, while “(“ means exclusive of the end point).
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for stack 1, we will use [0, n/3)
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for stack 2, we will use [n/3, 2n/3)
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for stack 3, we will use [2n/3, n)
This solution is based on the assumption that we do not have any extra information about
the usage of space by individual stacks and that we can’t either modify or use any extra
space. With these constraints, we are left with no other choice but to divide equally.
1 int stackSize = 300;
2 int[] buffer = new int [stackSize * 3];
3 int[] stackPointer = {0, 0, 0}; // stack pointers to track top elem
4
5 void push(int stackNum, int value) {
6 /* Find the index of the top element in the array + 1, and
7 * increment the stack pointer */
8 int index = stackNum * stackSize + stackPointer[stackNum] + 1;
9 stackPointer[stackNum]++;
10 buffer[index] = value;
11 }
12
13 int pop(int stackNum) {
14 int index = stackNum * stackSize + stackPointer[stackNum];
15 stackPointer[stackNum]--;
16 int value = buffer[index];
17 buffer[index]=0;
18 return value;
19 }
20
21 int peek(int stackNum) {
22 int index = stackNum * stackSize + stackPointer[stackNum];
23 return buffer[index];
24 }
25
26 boolean isEmpty(int stackNum) {
27 return stackPointer[stackNum] == stackNum*stackSize;
28 }
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Cracking the Coding Interview | Data Structures