LintCode: Binary Tree Postorder Traversal

C++，递归

/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 */
class Solution {
    /**
     * @param root: The root of binary tree.
     * @return: Postorder in vector which contains node values.
     */
public:
    vector<int> postorderTraversal(TreeNode *root) {
        // write your code here
        vector<int> result;
        if (root == NULL) {
            return result;
        }
        if (root->left != NULL) {
            vector<int> left = postorderTraversal(root->left);
            result.reserve(result.size() + left.size());
            result.insert(result.end(), left.begin(), left.end());
        }
        if (root->right != NULL) {
            vector<int> right = postorderTraversal(root->right);
            result.reserve(result.size() + right.size());
            result.insert(result.end(), right.begin(), right.end());
        }
        result.push_back(root->val);
        return result;
    }
};

C++，递归，辅助函数

/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 */
class Solution {
    /**
     * @param root: The root of binary tree.
     * @return: Postorder in vector which contains node values.
     */
public:
    vector<int> postorderTraversal(TreeNode *root) {
        // write your code here
        vector<int> result;
        if (root == NULL) {
            return result;
        } else {
            postorderCore(root, result);
        }
        return result;
    }
    void postorderCore(TreeNode *root, vector<int> &result) {
        if (root == NULL) {
            return;
        }
        if (root->left != NULL) {
            postorderCore(root->left, result);
        }
        if (root->right != NULL) {
            postorderCore(root->right, result);
        }
        result.push_back(root->val);
        return;
    } 
};

C++，非递归

［一个stack］

［一个cur指针］

［一个pre指针］

/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 */
class Solution {
    /**
     * @param root: The root of binary tree.
     * @return: Postorder in vector which contains node values.
     */
public:
    vector<int> postorderTraversal(TreeNode *root) {
        // write your code here
        vector<int> result;
        if (root == NULL) {
            return result;
        }
        
        TreeNode *cur = root, *pre = NULL;
        stack<TreeNode *> sta;
        
        while (cur != NULL || !sta.empty()) {
            while (cur != NULL) {
                sta.push(cur);
                cur = cur->left;
            }
            cur = sta.top();
            if (cur->right == NULL || cur->right == pre) {
                sta.pop();
                result.push_back(cur->val);
                pre = cur;
                cur = NULL;
            } else {
                cur = cur->right;
            }
        }
        return result;
    }
};