二叉树后序遍历

递归：


vector<int> vec;
 void process(TreeNode *root)
    {
        if(root == NULL)return ;
        process(root->left);
        process(root->right);
        vec.push_back(root->val);
        
    }

　　

非递归：运用前序遍历转换

    vector<int> postorderTraversal1(TreeNode *root) {
        stack<TreeNode*> s;
        vector<int> v;
        if(root == NULL)return v;
        s.push(root);
        while(!s.empty()){
            TreeNode *t = s.top();
            v.push_back(t->val);
            s.pop();
             if(t->left)s.push(t->left);
            if(t->right)s.push(t->right);
           
            
        }
        reverse(v.begin(),v.end());
        //process(root);
        return v;
    }

非递归


vector<int> postorderTraversal(TreeNode *root) {
            stack<TreeNode*> s;
            vector<int> v;
            if(root == NULL)return v;
            s.push(root);
            while(!s.empty()){
               TreeNode *t = s.top();
               while(t->left){s.push(t->left);t =t->left;}
               t = s.top();
                if(t->right == NULL)
                {
                    v.push_back(t->val);
                    s.pop();
                    if(s.empty())break;
                    TreeNode* r = s.top();
                    if( r->left == t)
                        r->left = NULL;
                    if(r->right == t)
                        r->right = NULL;
                }else{
                    s.push(t->right);
                }
                
            }
        return v;
    }