后序遍历二叉树

递归：

    vector<int> postorderTraversal(TreeNode *root) {        vector<int> result;        if(root==NULL)            return result;        PostCore(root,result);        return result;    }    void PostCore(TreeNode* root,vector<int>& result){        if(root->left){            PostCore(root->left,result);        }        if(root->right)            PostCore(root->right,result);        result.push_back(root->val);    }

非递归：

    vector<int> postorderTraversal(TreeNode *root) {        vector<int> result;        if(root==NULL)            return result;        stack<TreeNode*> trees;        trees.push(root);        while(!trees.empty()){            TreeNode* pNode=trees.top();            if(pNode->left==NULL&&pNode->right==NULL){                result.push_back(pNode->val);                trees.pop();            }            if(pNode->right){                trees.push(pNode->right);                pNode->right=NULL;            }            if(pNode->left){                trees.push(pNode->left);                pNode->left=NULL;            }        }        return result;    }

利用前序遍历：把根-》左-》右 变为 根-》右-》左  再把 结果 reverse

利用 栈

    vector<int> postorderTraversal(TreeNode *root) {        vector<int> result;        if(root==NULL)            return result;        stack<TreeNode*> trees;        trees.push(root);        while(!trees.empty()){            TreeNode* pNode=trees.top();            trees.pop();            result.push_back(pNode->val);            if(pNode->left)                trees.push(pNode->left);            if(pNode->right)                trees.push(pNode->right);        }        reverse(result.begin(),result.end());        return result;    }