Binary Tree | Inorder Traversal

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

Example 1:

Input: root = [1,null,2,3] Output: [1,3,2]

Example 2: Input: root = [] Output: []

Example 3: Input: root = [1] Output: [1]

Constraints: The number of nodes in the tree is in the range [0, 100]. -100 <= Node.val <= 100

Java Solution

**Approach: Morris Traversal**

  Definition for a binary tree node.
  public class TreeNode {
      int val;
      TreeNode left;
      TreeNode right;
      TreeNode() {}
      TreeNode(int val) { this.val = val; }
      TreeNode(int val, TreeNode left, TreeNode right) {
          this.val = val;
          this.left = left;
          this.right = right;
      }
  }

class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        TreeNode curr = root;
        TreeNode pre;
        while (curr != null) {
            if (curr.left == null) {
                res.add(curr.val);
                curr = curr.right; // move to next right node
            } else { // has a left subtree
                pre = curr.left;
                while (pre.right != null) { // find rightmost
                    pre = pre.right;
                }
                pre.right = curr; // put cur after the pre node
                TreeNode temp = curr; // store cur node
                curr = curr.left; // move cur to the top of the new tree
                temp.left = null; // original cur left be null, avoid infinite loops
            }
        }
        return res;
    }
}

C++ Solution

  Definition for a binary tree node.
  struct TreeNode {
      int val;
      TreeNode *left;
      TreeNode *right;
      TreeNode() : val(0), left(nullptr), right(nullptr) {}
      TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
      TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
  };

class Solution {
public:
    vector<int> inorderTraversal(TreeNode* root) {
        stack<TreeNode*> s;
        vector<int> res;
        TreeNode * node=root;
        while(!s.empty() || node!=NULL){
            while(node!=NULL){
                s.push(node);
                node=node->left;
            }
            node= s.top();
            s.pop();
            res.push_back(node->val);
            node=node->right;
        }
        return res;    
    }
};