Question

I want to print tree in zigzag order. Here is my code but it is not running. Can Someone point out the mistake ?
I have created two stacks to do the same, my algorithm terminates when two of them are empty, if any of them contains some nodes then these are printed and its childrens are pushed in the other stack. This process continues till both stacks become empty.

#include<stdio.h>
#define MAX_SIZE 100

struct TreeNode{
      int value;
      struct TreeNode *left;
      struct TreeNode *right;
};


struct TreeNode stackA[MAX_SIZE]; 
struct TreeNode stackB[MAX_SIZE];

int topA = 0;
int topB = 0;


void push(struct TreeNode stack[], struct TreeNode *node, int *top){
     if(*top > MAX_SIZE){
             printf("stack is full\n");
     }
     else{
          stack[*top] = *node;
          *top = *top +1;
     }
     return;
}

struct TreeNode* pop(struct TreeNode stack[], int *top){
     if(*top == 0){
             printf("stack is empty\n");
               return NULL;
     }
     else{
          struct TreeNode *tmp = stack + *top;
          *top = *top - 1;
          return tmp;
     }


}

int isEmpty(int *top){
        if(*top == 0){
                return 1;
        }

        return 0;
}
void printZigZag(struct TreeNode* root){
     push(stackA,  root,&topA);
     printf("%d\n", topA);
     while(!isEmpty(&topA) || !isEmpty(&topB)){
        while(!isEmpty(&topA)){
           struct TreeNode* temp = pop(stackA,&topA);
           printf("%d", temp->value);
           if(temp->left) push(stackB,temp->left,&topB);
           if(temp->right) push(stackB,temp->right,&topB);
           printf("%d %d",topA,topB);
           return;
       }
       while(!isEmpty(&topB)){
           struct TreeNode *temp = pop(stackB,&topB);
           printf("%d", temp->value);
           if(temp->right) push(stackA,temp->right,&topA);
           if(temp->left) push(stackA,temp->left,&topB);
      }
   }
}


int main(){
    struct TreeNode* root = (struct TreeNode *)malloc(sizeof(struct TreeNode));
    root->value = 5;
    root->left = NULL;
    root->right = NULL;

    struct TreeNode* first = (struct TreeNode *)malloc(sizeof(struct TreeNode));
    first->value = 15;
    first->left = NULL;
    first->right = NULL;
    root->left = first;


    struct TreeNode* second = (struct TreeNode *)malloc(sizeof(struct TreeNode));
    second->value = 235;
    second->left = NULL;
    second->right = NULL;
    root->right = second;


    struct TreeNode *third = (struct TreeNode *)malloc(sizeof(struct TreeNode));
    third->value = 45;
    third->left = NULL;
    third->right = NULL;
    first->left = third;


    struct TreeNode *fourth = (struct TreeNode *)malloc(sizeof(struct TreeNode));
    fourth->value = 55;
    fourth->left = NULL;
    fourth->right = NULL;
    first->right = fourth;

    printZigZag(root);

    system("pause");
    return 0;
}

Answer 1

I’ve check your code. The biggest mistake your have made is that the Pop function is implemented by error. The value of *pop should be decreased before you get the top value of the stack. Another mistake is that if(temp->left) push(stackA,temp->left,&topB) should be changed to if(temp->left) push(stackA,temp->left,&topA). Below is the code works, I change some small places to fit for c++.

#include<cstdio>
#define MAX_SIZE 100

struct TreeNode{
      int value;
      struct TreeNode *left;
      struct TreeNode *right;
};


struct TreeNode stackA[MAX_SIZE]; 
struct TreeNode stackB[MAX_SIZE];

int topA = 0;
int topB = 0;


void push(struct TreeNode stack[], struct TreeNode *node, int *top){
     if(*top > MAX_SIZE){
             printf("stack is full\n");
     }
     else{
          stack[*top] = *node;
          *top = *top +1;
     }
     return;
}

struct TreeNode* pop(struct TreeNode stack[], int *top){
     if(*top == 0){
             printf("stack is empty\n");
               return NULL;
     }
     else{
          *top = *top - 1; 
         struct TreeNode *tmp = stack +*top;         
          return tmp;
     }


}

int isEmpty(int *top){
        if(*top == 0){
                return 1;
        }

        return 0;
}
void printZigZag(struct TreeNode* root){
     push(stackA,  root,&topA);
     while(!isEmpty(&topA) || !isEmpty(&topB)){
        while(!isEmpty(&topA)){
           struct TreeNode* temp = pop(stackA,&topA);
           printf("%d\n", temp->value);
           if(temp->left) push(stackB,temp->left,&topB);
           if(temp->right) push(stackB,temp->right,&topB);
       }
       while(!isEmpty(&topB)){
           struct TreeNode *temp = pop(stackB,&topB);
           printf("%d\n", temp->value);
           if(temp->right) push(stackA,temp->right,&topA);
           if(temp->left) push(stackA,temp->left,&topA);
      }
   }
}

int main(){
    struct TreeNode* root = new TreeNode();
    root->value = 5;
    root->left = NULL;
    root->right = NULL;

    struct TreeNode* first =new TreeNode();
    first->value = 15;
    first->left = NULL;
    first->right = NULL;
    root->left = first;


    struct TreeNode* second = new TreeNode();
    second->value = 235;
    second->left = NULL;
    second->right = NULL;
    root->right = second;


    struct TreeNode *third = new TreeNode();
    third->value = 45;
    third->left = NULL;
    third->right = NULL;
    first->left = third;


    struct TreeNode *fourth = new TreeNode();
    fourth->value = 55;
    fourth->left = NULL;
    fourth->right = NULL;
    first->right = fourth;

    printZigZag(root);
    return 0;
}

Hope it helps!