Create and traverse a binary tree recursively in C

Question

I want to create a binary tree and traverse it by preorder traversal, and I use recursive method. These code can be compiled but can not run correctly, and I found it maybe can not finish the CreateBitree() function, but I don't know where the problem is.

#include <stdio.h>
#include <malloc.h>

typedef struct BiNode{
    int data;
    struct BiNode *lchild;
    struct BiNode *rchild;    //left and right child pointer
}BiNode;

int CreateBiTree(BiNode *T);
int TraverseBiTree(BiNode *T);

int main() {
    BiNode *t;
    CreateBiTree(t);
    TraverseBiTree(t);
    return 0;
}

int CreateBiTree(BiNode *T) {          //create a binary tree by preorder traversal
    char tmp;
    scanf("%c", &tmp);
    if(tmp == ' ')
    T = NULL;
    else {
        T = (BiNode *)malloc(sizeof(BiNode));
        T -> data = tmp;
        CreateBiTree(T -> lchild);
        CreateBiTree(T -> rchild);
    }
    return 1;
}

int TraverseBiTree(BiNode *T) {        //traverse a binary tree by preorder traversal
    if(T != NULL) {
        printf("%c\n", T -> data);
        TraverseBiTree(T -> lchild);
        TraverseBiTree(T -> rchild);
    }
    return 1;
}

For example, when I input a preorder sequence like "ABC##DE#G##F###"("#"means space), and then it still let me to input, I think the TraverseBiTree() function hasn't been executed.

When you say it doesn't work... What do you mean by that? Wrong out put? Segmentation fault? Something else? — FDinoff
– FDinoff, Commented May 19, 2013 at 4:18
You're passing your pointer values into CreateBiTree by value. The caller-side of that is never getting updated pointers (including itself). — WhozCraig
– WhozCraig, Commented May 19, 2013 at 4:19

perreal · Accepted Answer · 2013-05-19 05:08:33Z

An assignment of a pointer value to a pointer within a function does not have any effect outside the scope of that function. Doing this:

int CreateBiTree(BiNode *T) { 
  /* ... */
  T = NULL;

is same as doing this:

int func(int i) { 
  /* ... */
  i = 0;

A pointer to the argument is necessary in these cases:

int CreateBiTree(BiNode **T) { 
  /* ... */
  T[0] = NULL;  // or... *T = NULL;

With some changes to the initial code:

int main() {
    BiNode *t; 
    CreateBiTree(&t);
    TraverseBiTree(t);
    return 0;
}

int CreateBiTree(BiNode **T) {          //create a binary tree by preorder traversal
    char tmp;
    scanf("%c", &tmp);
    if(tmp == ' ')
    T[0] = NULL;
    else {
        T[0] = (BiNode *)malloc(sizeof(BiNode));
        T[0]-> data = tmp;
        CreateBiTree(&(T[0]->lchild));
        CreateBiTree(&(T[0]->rchild));
    }   
    return 1;
}

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