Binary Tree Postorder Traversal

http://www.lintcode.com/en/problem/binary-tree-postorder-traversal/

Solution

Recursion.

1. 访问左子树
2. 访问右子树
3. 输出根

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

Iteration.

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
        stack<TreeNode*> mystk;
        vector<int> result;
        if(root == NULL) 
            return result;
        mystk.push(root);
        TreeNode*cur, *pre;
        cur = root;
        pre = root;
        while (!mystk.empty()) {
            cur = mystk.top();
            if((cur->left == NULL && cur->right == NULL) || cur->right==pre
                || cur->left==pre) {
                mystk.pop();
                result.push_back(cur->val);
                pre = cur;
            } else {
                if(cur->right)
                    mystk.push(cur->right);
                if(cur->left)
                    mystk.push(cur->left);
            }
        }
    }
};

Morrise traversal O(1) space

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> res;
        TreeNode dummy(INT_MIN);
        dummy.left = root;
        TreeNode *curr = &dummy;
        while (curr) {
            if (!curr->left) {
                curr = curr->right;
            } else {
                TreeNode *node = curr->left;
                while (node->right && node->right != curr) {
                    node = node->right;
                }
                if (!node->right) {
                    node->right = curr;
                    curr = curr->left;
                } else {
                    vector<int> v = trace_back(curr->left, node);
                    res.insert(res.end(), v.begin(), v.end());
                    node->right = nullptr;
                    curr = curr->right;
                }
            }
        }
        return res;
    }

    vector<int> trace_back(TreeNode *frm, TreeNode *to) {
        vector<int> res;
        TreeNode *curr = frm;
        while (curr != to) {
            res.emplace_back(curr->val);
            curr = curr->right;
        }
        res.emplace_back(to->val);
        reverse(res.begin(), res.end());
        return res;
    }
};