c - pointer changes value without being passed as argument -

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following code snippet code involving transformations on binary tree. 

void fixprevptr(struct node *root) {     static struct node *pre = null;      if (root != null)     {         fixprevptr(root->left);         root->left = pre;         pre = root;         fixprevptr(root->right);     } }

here 'pre' initialised in every function null. when function entered 'if' clause , root->left=pre, executed, pre being assigned not null. somehow changed function fixprevptr(root->left).

my question how changed without being passed function.

thanks in advance.

pre static keeps it's value call call. fixprevptr() recursive (calls itself) change pre "sticks".


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