Binary Tree Iterative Traversal

Preorder

 class Solution {
 public:
     vector<int> preorderTraversal(TreeNode* root) {
         vector<int> res;
         if (!root) return res;
         stack<TreeNode *> sta;
         sta.push(root);
         while (!sta.empty())
         {
             TreeNode* cur = sta.top();
             sta.pop();
             res.push_back(cur->val);
             if (cur->right) sta.push(cur->right);
             if (cur->left) sta.push(cur->left);
         }
         return res;
     }
 };

postorder

 class Solution {
 public:
     vector<int> postorderTraversal(TreeNode* root) {
         vector<int> res;
         if (!root) return res;
         stack<TreeNode *> sta;
         sta.push(root);
         TreeNode *last = NULL;
         while (!sta.empty())
         {
             TreeNode *cur = sta.top();
             TreeNode *left = cur->left, *right = cur->right;
             if (left && (!last || (last != left && last != right)))
                 sta.push(left);
             else if (right && (!last || last != right))
                 sta.push(right);
             else
             {
                 res.push_back(cur->val);
                 last = cur;
                 sta.pop();
             }
         }
         return res;
     }
 };

inorder

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