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LeetCode-Binary Tree Postorder Traversal

使用了两种方法对二叉树进行后序遍历，具体可以参考二叉树相关知识点总结。

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/**
 * 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;
        postorder2(root,result);
        return result;
    }
private:
    void postorder1(TreeNode* root, vector<int>& result){
        if(root==nullptr) return;
        postorder1(root->left,result);
        postorder1(root->right,result);
        result.push_back(root->val);
    }
    void postorder2(TreeNode* root, vector<int>& result){
        if(root==nullptr) return;
        stack<TreeNode*> s;
        stack<int> res;
        s.push(root);
        TreeNode* n;
        while(!s.empty()){
            n=s.top();
            s.pop();
            res.push(n->val);
            if(n->left) s.push(n->left);
            if(n->right) s.push(n->right);
        }
        while(!res.empty()){
            int t=res.top();
            res.pop();
            result.push_back(t);
        }
    }
};