LeetCode: Binary Tree Inorder Traversal, Morris In Order Traversal

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?

Morris Inorder Traversal, no need to use stack.

Algorithm:

Traverse left subtree,

visit node,

Traverse right subtree

In Morris Inorder Traversal, need to figure out after traversed left-subtree, which node is the next to visit

If left-node is empty, visit the node and traverse the right subtree

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> inorderTraversal(TreeNode *root) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        TreeNode *p, *pre
        vector<int> ret;
        for(p=root; p!=NULL;){
            if(p->left == NULL){ //traverse the node and then traverse the right subtree
                ret.push_back(p->val);
                p = p->right;
                continue;
            }
            for(pre = p->left; pre->right != NULL && pre->right != p; pre=pre->right);
            if(pre->right == NULL){ //need to figure out after traversed the left subtree of this node, it could return to visit this node
                pre->right = p;
                p = p->left;
            }else{ //traverse the node and traverse the right subtree
                ret.push_back(p->val); 
                p = p->right;
                pre->right = NULL;
            }
        }
        return ret;
    }
};

Reference:

http://comsci.liu.edu/~murali/algo/Morris.htm