[LeetCode] Binary Tree Postorder Traversal

Given a binary tree, return the postorder traversal of its nodes’ values.

For example:
Given binary tree {1,#,2,3},

return [3,2,1].

Note: Recursive solution is trivial, could you do it iteratively?

递归实现代码

/*****************************************************************
    *  @Author   : 楚兴
    *  @Date     : 2015/2/24 01:27
    *  @Status   : Accepted
    *  @Runtime  : 4 ms
******************************************************************/
struct TreeNode
{
    int val;
    TreeNode *left;
    TreeNode *right;
    TreeNode(int x) : val(x), left(NULL), right(NULL){}
};

class Solution {
public:
   void helper(TreeNode *root, vector<int> &result) {
        if (root)
        {
            helper(root->left, result);
            helper(root->right, result);
            result.push_back(root->val);
        }
    }

    vector<int> postorderTraversal(TreeNode *root) {
        vector<int> result;
        helper(root, result);

        return result;
    }
};

非递归实现代码1

class Solution {
public:
    vector<int> postorderTraversal(TreeNode *root) {
        vector<int> result;
        if (root == NULL)
        {
            return result;
        }
        list<TreeNode*> nodes;
        TreeNode *p = root;
        TreeNode *cur;
        nodes.push_back(p);
        while(!nodes.empty())
        {
            cur = nodes.front();
            //①当前结点不是叶子结点，但是孩子已经遍历过。
            //②当前结点是叶子结点
            if (cur->right == p || cur->left == p ||(cur->left == NULL && cur->right == NULL))
            {
                result.push_back(cur->val);
                nodes.pop_front();
                p = cur;
            }
            else
            {
                if (cur->right != NULL)
                {
                    nodes.push_front(cur->right);
                }
                if (cur->left != NULL)
                {
                    nodes.push_front(cur->left);
                }
            }
        }

        return result;
    }
};

非递归实现代码2

class Solution {
public:
    vector<int> postorderTraversal(TreeNode *root) {
        vector<int> result;
        stack<TreeNode*> nodes;
        TreeNode *p = NULL;
        TreeNode *cur = root;
        while(cur != NULL || !nodes.empty())
        {
            if (cur != NULL)
            {
                while (cur != NULL)
                {
                    nodes.push(cur);
                    cur = cur->left;
                }
            }
            else
            {
                cur = nodes.top();
                if (cur->right == NULL || cur->right == p)
                {
                    nodes.pop();
                    p = cur;
                    result.push_back(cur->val);
                    cur = NULL;
                }
                else
                {
                    cur = cur->right;
                }
            }
        }

        return result;
    }
};

先序遍历的实现见博文Binary Tree Preorder Traversal