How to implement 3 stacks with one array?

Question

Sometimes, I come across the following interview question: How to implement 3 stacks with one array ? Of course, any static allocation is not a solution.

Answer 1

package job.interview; import java.util.Arrays;

public class NStack1ArrayGen<T> {

    T storage[];
    int numOfStacks;
    Integer top[];
    public NStack1ArrayGen(int numOfStks, T myStorage[]){
        storage = myStorage;
        numOfStacks = numOfStks;
        top = new Integer[numOfStks];
        for(int i=0;i<numOfStks;i++){top[i]=-1;}
    }
    public void push(int stk_indx, T value){
        int r_indx = stk_indx -1;
        if(top[r_indx]+numOfStacks < storage.length){
            top[r_indx] = top[r_indx] < 0 ? stk_indx-1 : top[r_indx]+numOfStacks;
            storage[top[r_indx]] = value;
        }
    }
    public T pop(int stk_indx){
        T ret = top[stk_indx-1]<0 ? null : storage[top[stk_indx-1]];
        top[stk_indx-1] -= numOfStacks;
        return ret;
    }
    public void printInfo(){
        print("The array", Arrays.toString(storage));
        print("The top indices", Arrays.toString(top));
        for(int j=1;j<=numOfStacks;j++){
            printStack(j);
        }
    }
    public void print(String name, String value){
        System.out.println(name + " ==> " + value);
    }
    public void printStack(int indx){
        String str = "";
        while(top[indx-1]>=0){
            str+=(str.length()>0 ? "," : "") + pop(indx);
        }
        print("Stack#"+indx,str);
    }
    public static void main (String args[])throws Exception{
        int count=4, tsize=40;
        int size[]={105,108,310,105};
        NStack1ArrayGen<String> mystack = new NStack1ArrayGen<String>(count,new String[tsize]); 
        for(int i=1;i<=count;i++){
            for(int j=1;j<=size[i-1];j++){
                mystack.push(i, "stk"+i+"_value"+j);
            }
        }
    }
}

This prints:

The array ==> [stk1_value1, stk2_value1, stk3_value1, stk4_value1, stk1_value2, stk2_value2, stk3_value2, stk4_value2, stk1_value3, stk2_value3, stk3_value3, stk4_value3, stk1_value4, stk2_value4, stk3_value4, stk4_value4, stk1_value5, stk2_value5, stk3_value5, stk4_value5, stk1_value6, stk2_value6, stk3_value6, stk4_value6, stk1_value7, stk2_value7, stk3_value7, stk4_value7, stk1_value8, stk2_value8, stk3_value8, stk4_value8, stk1_value9, stk2_value9, stk3_value9, stk4_value9, stk1_value10, stk2_value10, stk3_value10, stk4_value10] The top indices ==> [36, 37, 38, 39] Stack#1 ==> stk1_value10,stk1_value9,stk1_value8,stk1_value7,stk1_value6,stk1_value5,stk1_value4,stk1_value3,stk1_value2,stk1_value1 Stack#2 ==> stk2_value10,stk2_value9,stk2_value8,stk2_value7,stk2_value6,stk2_value5,stk2_value4,stk2_value3,stk2_value2,stk2_value1 Stack#3 ==> stk3_value10,stk3_value9,stk3_value8,stk3_value7,stk3_value6,stk3_value5,stk3_value4,stk3_value3,stk3_value2,stk3_value1 Stack#4 ==> stk4_value10,stk4_value9,stk4_value8,stk4_value7,stk4_value6,stk4_value5,stk4_value4,stk4_value3,stk4_value2,stk4_value1