Mirror a binary tree in C.

tree.c

#include <assert.h>
#include <stdio.h>

#define swap(x, y) { void* tmp = x; x = y; y = tmp; }

typedef struct node_t* tree_t;

typedef struct node_t {
  int elem;
  tree_t left, right;
} node_t;

void invert_tree(tree_t* tree) {
  if(!*tree) return;
  invert_tree(&(*tree)->left);
  invert_tree(&(*tree)->right);
  swap((*tree)->left, (*tree)->right);
}

void print_tree(tree_t tree) {
  if(!tree) return;
  print_tree(tree->left);
  printf("%d ", tree->elem);
  print_tree(tree->right);
}

int main(void) {
  // Test tree:
  //
  //     4           4
  //    / \         / \
  //   2   5  ==>  5   2
  //  / \             / \
  // 1   3           3   1
  //
  node_t _1 = { 1, NULL, NULL };
  node_t _3 = { 3, NULL, NULL };
  node_t _2 = { 2, &_1, &_3 };
  node_t _5 = { 5, NULL, NULL };
  node_t _4 = { 4, &_2, &_5 };
  tree_t root = &_4;

  print_tree(root);
  invert_tree(&root);
  print_tree(root);
  return 0;
}