Mirror a binary tree


Mirroring a binary tree, a common question nowadays to understand whether the candidate knows about tree traversal.

Correct code for creating mirrored tree, in-place
- do a post order traversal to read the full tree into the recursion stack
- do the primitive swap operation
https://www.geeksforgeeks.org/write-an-efficient-c-function-to-convert-a-tree-into-its-mirror-tree/
(1)  Call Mirror for left-subtree    i.e., Mirror(left-subtree)
(2)  Call Mirror for right-subtree  i.e., Mirror(right-subtree)
(3)  Swap left and right subtrees.
          temp = left-subtree
          left-subtree = right-subtree
          right-subtree = temp
    Node mirror(Node node)
    {
        if (node == null)
            return node;
  
        /* do the subtrees */
        Node left = mirror(node.left);
        Node right = mirror(node.right);
  
        /* swap the left and right pointers */
        node.left = right;
        node.right = left;
  
        return node;
    }

http://analgorithmaday.blogspot.com/2013/05/mirror-binary-tree.html
Tree* mirror(Tree* node)
{
    if(node == NULL)
        return NULL;
 
    mirror(node->left);
    mirror(node->right);
    Tree *tmp = node->left;
    node->left = node->right;
    node->right = tmp;
 
    return node;
}
Corrected code for reproduction of mirrored tree, as a copy
Tree* mirror(Tree* node)
{
    if(node == NULL)
        return NULL;
 
    Tree *newNode = createNode(node->data);
    newNode->left = mirror(node->right);
    newNode->right = mirror(node->left);
 
    return newNode;
}
struct node
02{
03    int value;
04    struct node* left;
05    struct node* right;
06};
07
08
09struct node *mirrorTree(struct node *root)
10{
11  struct node *temp;
12   
13  if(root==NULL)
14    return(NULL);
15    
16  temp = (struct node *) malloc(sizeof(struct node));
17  temp->value = root->value;
18
19  temp->left  = mirrorTree(root->right);
20  temp->right = mirrorTree(root->left);
21
22  return(temp);
23}
Correct code for creating mirrored tree, in-place
- do a post order traversal to read the full tree into the recursion stack
Tree* mirror(Tree* node)
{
    if(node == NULL)
        return NULL;
 
    mirror(node->left);
    mirror(node->right);
    Tree *tmp = node->left;
    node->left = node->right;
    node->right = tmp;
 
    return node;
}

Iteratively mirror a tree
http://haixiaoyang.wordpress.com/2012/07/14/iteratively-mirror-a-tree/
void IterativeMirror(NODE* pRoot)
{
    if (NULL == pRoot)
        return;
    stack<NODE*> stk;
    stk.push(pRoot);
    while (!stk.empty())
    {
        NODE* pTop = stk.top();
        stk.pop();
        swap(pTop->pLft, pTop->pRgt);
        if (pTop->pLft != NULL)
            stk.push(pTop->pLft);
        if (pTop->pRgt != NULL)
            stk.push(pTop->pRgt);
    }
}
https://github.com/yadongwen/Company1-InterviewQuestions-Answers-Java/blob/master/mirror%20of%20binary%20tree.java
The order(swap or put to queue) doesn't matter.
    public static void mirrorIterative(TreeNode t) {
        if(t == null) return;
        LinkedList<TreeNode> q = new LinkedList<TreeNode>();
        q.add(t);
        while(!q.isEmpty()) {
            TreeNode tr = q.poll();
            if(tr.left != null) q.add(tr.left);
            if(tr.right != null) q.add(tr.right);
            TreeNode temp = tr.left;
            tr.left = tr.right;
            tr.right = temp;
        }
    }

void tree_mirror(struct node* node)
02{
03   struct node *temp;
04
05   if (node==NULL)
06   {
07     return;
08   }
09   else
10   {
11      tree_mirror(node->left);
12      tree_mirror(node->right);
13
14      // Swap the pointers in this node
15      temp = node->left;
16      node->left = node->right;
17      node->right = temp;
18   }
19}
http://algorithmproblems.blogspot.com/2012/12/are-two-binary-trees-mirror-image-of.html
Are two Binary Trees mirror image of each other?
boolean areMirrorTrees(BinaryTree a, BinaryTree b) {
    if (a == null && b == null) {
        return true;
    }
    if (a == null || b == null) {
        return false;
    }
    return a.data == b.data && areMirrorTrees(a.left, b.right)
            && areMirrorTrees(a.right, b.left);
}
http://www.acmerblog.com/offer11-2964.html
10typedef struct Node {
11    struct Node * l;
12    struct Node * r;
13    int data;
14}* pNode, Node;
15char temp[2];
16Node tree[1001];
17//先右后左输出二叉树
18void print(pNode root) {
19    if (root == 0)
20        return;
21    printf(" %d", root->data);
22    print(root->r);
23    print(root->l);
24}
25int main() {
26    while (~scanf("%d", &n)) {
27        for ( i = 1; i <= n; i++) {
28            scanf("%d", &tree[i].data);
29            tree[i].l = tree[i].r = 0;
30        }
31        for ( i = 1; i <= n; i++) {
32            scanf("%s %d", temp, &a);
33            if (temp[0] == 'd') {
34                tree[i].l = &tree[a];
35                scanf("%d", &a);
36                tree[i].r = &tree[a];
37            }else if(temp[0] == 'l')
38                tree[i].l = &tree[a];
39            else if(temp[0] == 'r')
40                tree[i].r = &tree[a];
41        }
42        if (n) {
43            printf("%d", tree[1].data);//由于空格判断,根节点单独输出
44            print(tree[1].r);
45            print(tree[1].l);
46            puts("");
47        else
48            puts("NULL");
49    }
50    return 0;
51}
Read full article from An Algorithm a Day: Mirror a binary tree

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