LeetCode：145_Binary Tree Postorder Traversal | 二叉树后序遍历 | Hard

题目：Binary Tree Postorder Traversal

二叉树的后序遍历，题目要求是采用非递归的方式，这个在上数据结构的课时已经很清楚了，二叉树的非递归遍历不管采用何种方式，都需要用到栈结构作为中转，代码很简单，见下：

struct TreeNode {
    int            val;
    TreeNode*    left;
    TreeNode*    right;
    TreeNode(int x): val(x), left(NULL),right(NULL) {}
};

vector<int> preorderTraversal(TreeNode *root) //非递归的后序遍历(用栈实现)
{
    if (NULL == root) {
        return vector<int>();
    }
    
    stack<TreeNode *> tree_stack;
    vector<int> tree_vector;

    tree_stack.push(root);
    TreeNode *p = NULL;
    while (!tree_stack.empty()) {
        TreeNode *pTemp = tree_stack.top();
        if ((pTemp->left == NULL && pTemp->right == NULL) || ((p != NULL) && (pTemp->left == p || pTemp->right == p))) {
            tree_vector.push_back(pTemp->val);
            tree_stack.pop();
            
            p = pTemp;
        }
        else {
            while(pTemp) {
                if (pTemp->right != NULL) 
                    tree_stack.push(pTemp->right);

                if (pTemp->left != NULL)
                    tree_stack.push(pTemp->left);
                pTemp = pTemp->left;
            }
        }
    }
    return tree_vector;
}