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

For example:
Given binary tree {1,#,2,3},

   1
\
2
/
3

return [3,2,1].

Note: Recursive solution is trivial, could you do it iteratively?

class Solution {
public:
vector<int> postorderTraversal(TreeNode *root) {
vector<int> result;
if(!root) return result; stack<MyNode*> treeStack;
MyNode* myRoot = new MyNode(root);
MyNode* current, *newNode;
treeStack.push(myRoot); while(!treeStack.empty())
{
current = treeStack.top();
treeStack.pop();
if(!current->flag)
{
current->flag = true;
treeStack.push(current);
if(current->node->right) {
newNode = new MyNode(current->node->right);
treeStack.push(newNode);
}
if(current->node->left) {
newNode = new MyNode(current->node->left);
treeStack.push(newNode);
}
}
else
{
result.push_back(current->node->val);
}
}
return result; }
struct MyNode
{
TreeNode* node;
bool flag; //indicate if the node has been visited
MyNode(TreeNode* x) : node(x),flag(false) {}
};
};

法II: 不重新定义结构,以root->right->left的顺序访问节点,最后逆序。

/**
* 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> postorderTraversal(TreeNode* root) {
vector<int> result;
if(!root) return result; stack<TreeNode*> treeStack;
TreeNode* current;
treeStack.push(root); //visit in order root->right->left
while(!treeStack.empty())
{
current = treeStack.top();
treeStack.pop();
result.push_back(current->val);
if(current->left) treeStack.push(current->left);
if(current->right) treeStack.push(current->right);
} //reverse result
int size = result.size();
int hsize = size>>;
int tmp;
for(int i = ; i < hsize; i++){
tmp = result[i];
result[i]=result[size--i];
result[size--i]=tmp;
}
return result;
}
};

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