leetcode Binary Tree Postorder Traversal

题目要求利用迭代算法，也即非递归方法实现二叉树的后序遍历

算法中主要利用了栈

代码

//迭代算法 后序遍历 vector<int> postorderTraversal(TreeNode *root) {          vector<int> res;stack<TreeNode*> stackTreeNodes;if(root == NULL)return res;stackTreeNodes.push(root);    TreeNode *pre, *cur;pre = NULL;cur = root;while(!stackTreeNodes.empty()){cur = stackTreeNodes.top();if((cur->left==NULL&&cur->right==NULL)||((pre!=NULL)&&(pre == cur->left||pre == cur->right))){res.push_back(cur ->val);stackTreeNodes.pop();pre = cur;}else{if(cur->right!=NULL)stackTreeNodes.push(cur->right);if(cur->left!=NULL)stackTreeNodes.push(cur->left);}}return res;    }  

先序与中序的非递归算法如下

//迭代算法 先序遍历vector<int> preorderTraversal(TreeNode * root){vector<int> res;stack<TreeNode *>stackTreeNodes;if(root==NULL)return res;TreeNode * cur;cur = root;stackTreeNodes.push(root);while(!stackTreeNodes.empty()){cur = stackTreeNodes.top();res.push_back(cur->val);stackTreeNodes.pop();if(cur->right!=NULL)stackTreeNodes.push(cur->right);if(cur->left!=NULL)stackTreeNodes.push(cur->left);}return res;}

//迭代算法 中序遍历vector<int> inorderTraversal(TreeNode * root){vector<int> res;stack<TreeNode *> stackTreeNodes;TreeNode * cur = root;while(!stackTreeNodes.empty()||cur!=NULL){while(cur!=NULL){stackTreeNodes.push(cur);cur = cur->left;}if(!stackTreeNodes.empty()){cur = stackTreeNodes.top();res.push_back(cur->val);stackTreeNodes.pop();cur = cur->right;}}return res;}