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Binary Tree Inorder Traversal

描述​

Given a binary tree, return the inorder traversal of its nodes' values.

For example:

Given binary tree {1,#,2,3},

 1
  \
   2
  /
 3

return [1,3,2].

Note: Recursive solution is trivial, could you do it iteratively?

分析​

用栈或者 Morris 遍历。

栈​

// Binary Tree Inorder Traversal
// 使用栈，时间复杂度O(n)，空间复杂度O(n)
class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        ArrayList<Integer> result = new ArrayList<>();
        Stack<TreeNode> s = new Stack<>();
        TreeNode p = root;

        while (!s.empty() || p != null) {
            if (p != null) {
                s.push(p);
                p = p.left;
            } else {
                p = s.pop();
                result.add(p.val);
                p = p.right;
            }
        }
        return result;
    }
}

Morris 中序遍历​

// Binary Tree Inorder Traversal
// Morris中序遍历，时间复杂度O(n)，空间复杂度O(1)
class Solution {
public:
    vector<int> inorderTraversal(TreeNode *root) {
        vector<int> result;
        TreeNode *cur = root, *prev = nullptr;

        while (cur != nullptr) {
            if (cur->left == nullptr) {
                result.push_back(cur->val);
                prev = cur;
                cur = cur->right;
            } else {
                /* 查找前驱 */
                TreeNode *node = cur->left;
                while (node->right != nullptr && node->right != cur)
                    node = node->right;

                if (node->right == nullptr) { /* 还没线索化，则建立线索 */
                    node->right = cur;
                    /* prev = cur; 不能有这句，cur还没有被访问 */
                    cur = cur->left;
                } else {    /* 已经线索化，则访问节点，并删除线索  */
                    result.push_back(cur->val);
                    node->right = nullptr;
                    prev = cur;
                    cur = cur->right;
                }
            }
        }
        return result;
    }
};

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