For two trees to be mirror images , the following three conditions must be true 1 - Their root node 's  key must be same 2 - left  subtree of left tree and right subtree  of right tree have to be mirror images  3 - right subtree of left tree and left subtree  of right tree have to be mirror images
if ( root1 && root2 && root1 -> key == root2 -> key ) return isMirror ( root1 -> left , root2 -> right ) && isMirror ( root1 -> right , root2 -> left ) ;