invert_binary_tree.c

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     struct TreeNode *left;
 *     struct TreeNode *right;
 * };
 */

#define swap_nodes(a, b)    \
({                          \
    a = (((struct TreeNode *) ((uintptr_t) (a) ^ (uintptr_t) (b)))); \
    b = (((struct TreeNode *) ((uintptr_t) (a) ^ (uintptr_t) (b)))); \
    a = (((struct TreeNode *) ((uintptr_t) (a) ^ (uintptr_t) (b)))); \
})

struct TreeNode* invertTree(struct TreeNode* root) {
    if (!root)
        return NULL;

    invertTree(root->left);
    invertTree(root->right);

    swap_nodes(root->left, root->right);

    return root;
}