Morris遍历

Morris遍历最主要的特点是不需要堆栈，其空间复杂度为O(1)，算法的核心步骤在于，找到当前节点左子树的最右节点，将该节点的右指针指向当前节点，从而在进入左子树之后，可以循此虚拟的连接回到当前节点。在左子树访问结束后，还要消除该虚拟连接，恢复原二叉树的结构。

Morris遍历的C++代码结构如下：

TreeNode *cur, *prev;cur = root;while (cur != nullptr) {if (cur->left == nullptr) {//do sth.prev = cur; /* */cur = cur->right;} else {TreeNode *node = cur->left;while (node->right != nullptr && node->right != cur)node = node->right;if (node->right == nullptr) { //visit curnode->right = cur;//prev = cur; //only if cur is just visited herecur = cur->left;} else { /*left has finished*/node->right = nullptr;// prev = cur; //only if cur is just visited herecur = cur->right;}}}