【Leetcode】【Medium】Binary Tree Inorder 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?

解题思路：

二叉树中序非递归遍历，使用栈来保存遍历到的但是还没有访问其右子树元素的结点；

代码：

/**
 * Definition for a binary tree node.
 * 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) {
        vector<int> vals;
        stack<TreeNode*> nodes;
        TreeNode* node = root;
        
        while (node != NULL || !nodes.empty()) {
            while (node) {
                nodes.push(node);
                node = node->left;
            }
            
            node = nodes.top();
            nodes.pop();
            vals.push_back(node->val);
            node = node->right;
        }
        
        return vals;
    }
};