Morris中序遍历

1.用途：将中序遍历的空间复杂度降为O(1)

2.实现：

void Morris(root){
    struct TreeNode *p = root, *pre = NULL;
    while (p){
        if (!p->left){                                                  //当前结点左子树为空
            Visit(p);                                                   //访问p结点
            p = p->right;                                               //移动到当前结点的右子树
            continue;
        }
        pre = p->left;                                                  //当前结点左子树非空
        while (pre->right && pre->right != p) pre = pre->right;         //pre指向当前结点的前驱结点
        if (!pre->right){                                               //把前驱结点链接到当前结点
            pre->right = p;
            p = p->left;
        } else {                                                        //断开前驱结点与其后继结点
            pre->right = NULL;
            Visit(pre);
            p = p->right;
        }
    }
}