[LeetCode] 94. Binary Tree Inorder Traversal 二叉树的中序遍历

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?

confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.

 

二叉树的中序遍历顺序为左-根-右，可以有递归和非递归来解，非递归解法又分为两种，一种是使用栈来接，另一种不需要使用栈的Morris方法。

Morris方法可参考帖子：Morris Traversal方法遍历二叉树（非递归，不用栈，O(1)空间）

Java:  Without Recursion, using Stack

public class Solution {
    /**
     * @param root: The root of binary tree.
     * @return: Inorder in ArrayList which contains node values.
     */
    public ArrayList<Integer> inorderTraversal(TreeNode root) {
        Stack<TreeNode> stack = new Stack<TreeNode>();
        ArrayList<Integer> result = new ArrayList<Integer>();
        TreeNode curt = root;
        while (curt != null || !stack.empty()) {
            while (curt != null) {
                stack.add(curt);
                curt = curt.left;
            }
            curt = stack.pop();
            result.add(curt.val);
            curt = curt.right;
        }
        return result;
    }
}

Python: Recursion, Time: O(n), Space: O(h) # h is the height of tree.

class Solution(object):
    def inorderTraversal(self, root):
        res = []
        if root:
            res = self.inorderTraversal(root.left) 
            res.append(root.val)
            res = res + self.inorderTraversal(root.right)
        return res

Python: Recursion, briefly done

class Solution(object):
    def inorderTraversal(self, root):
        return (self.inorderTraversal(root.left) + [root.val] + self.inorderTraversal(root.right)) if root else []

Pyhton: Without recursion, using stack,  Time: O(n), Space: O(h)  # h is the height of tree.

class Solution2(object):
    def inorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        result, stack = [], [(root, False)]
        while stack:
            root, is_visited = stack.pop()
            if root is None:
                continue
            if is_visited:
                result.append(root.val)
            else:
                stack.append((root.right, False))
                stack.append((root, True))
                stack.append((root.left, False))
        return result

Python:  Morris Inorder Traversal, Without recursion, Without stack,  Time: O(n), Space: O(1)

class Solution(object):
    def inorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        result, curr = [], root
        while curr:
            if curr.left is None:
                result.append(curr.val)
                curr = curr.right
            else:
                node = curr.left
                while node.right and node.right != curr:
                    node = node.right
            
                if node.right is None:
                    node.right = curr
                    curr = curr.left
                else:
                    result.append(curr.val)
                    node.right = None
                    curr = curr.right
                
        return result

C++: Recursion

class Solution {
    /**
     * @param root: The root of binary tree.
     * @return: Inorder in vector which contains node values.
     */
public:
    vector<int> inorder;

    void traverse(TreeNode *root) {
        if (root == NULL) {
            return;
        }
        traverse(root->left);
        inorder.push_back(root->val);
        traverse(root->right);
    }

    vector<int> inorderTraversal(TreeNode *root) {
        inorder.clear();
        traverse(root);
        return inorder;
    }
};

C++: Using Stack.

class Solution2 {
public:
    vector<int> inorderTraversal(TreeNode* root) {
        vector<int> res;
        stack<pair<TreeNode *, bool>> s;
        s.emplace(root, false);
        while (!s.empty()) {
            bool visited;
            tie(root, visited) = s.top();
            s.pop();
            if (root == nullptr) {
                continue;
            }
            if (visited) {
                res.emplace_back(root->val);
            } else {
                s.emplace(root->right, false);
                s.emplace(root, true);
                s.emplace(root->left, false);
            }
        }
        return res;
    }
};

C++: Morris

class Solution {
public:
    vector<int> inorderTraversal(TreeNode* root) {
        vector<int> res;
        TreeNode *curr = root;
        while (curr) {
            if (!curr->left) {
                res.emplace_back(curr->val);
                curr = curr->right;
            } else {
                TreeNode *node = curr->left;
                while (node->right && node->right != curr) {
                    node = node->right;
                }
                if (!node->right) {
                    node->right = curr;
                    curr = curr->left;
                } else {
                    res.emplace_back(curr->val);
                    node->right = nullptr;
                    curr = curr->right;
                }
            }
        }
        return res;
    }
};

类似题目：

[LeetCode] 144. Binary Tree Preorder Traversal 二叉树的先序遍历

[LeetCode] 145. Binary Tree Postorder Traversal 二叉树的后序遍历