145. Binary Tree Postorder Traversal

• 后序遍历二叉树(非递归实现)
  题目来源

• C++代码实现

class Solution
{
public:
	vector<int> postorderTraversal(TreeNode* root)
	{
		vector<int> result;
		stack<TreeNode*> datastack1;
		stack<TreeNode*> datastack2;

		if (root != NULL)
		{
			datastack1.push(root);

			while (!datastack1.empty())
			{
				root = datastack1.top();
				datastack2.push(root);
				datastack1.pop();
				if (root->left != NULL)
				{
					datastack1.push(root->left);
				}
				if (root->right != NULL)
				{
					datastack1.push(root->right);
				}
			}
		}
		int length = datastack2.size();
		for (int i = 0; i < length; i++)
		{
			result.push_back((datastack2.top())->val);
			datastack2.pop();
		}

		return result;
	}
};

将最后的datastack2向result中倒数据改为如下代码也可：

        while(!datastack2.empty())
        {
            result.push_back( (datastack2.top() )->val);
            datastack2.pop();
        }
		

• 第一次出现BUG:

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution 
{
public:
    vector<int> postorderTraversal(TreeNode* root)
	{
		vector<int> result;
        stack<TreeNode*> datastack1;
		stack<TreeNode*> datastack2;
		
        if(root != NULL)
		{
			datastack1.push(root);
			
			while(!datastack1.empty())
			{
				root = datastack1.top();
				datastack2.push(root);
				datastack1.pop();
				if(root->left != NULL)
				{
					datastack1.push(root->left);
				}
				if(root->right != NULL)
				{
					datastack1.push(root->right);
				}
			}
		}
		
		for(int i = 0;i < datastack2.size();i++)
		{
			result.push_back( (datastack2.top() )->val);
			datastack2.pop();
		}
		
		return result;
    }
};

最后发现BUG出现在

		for(int i = 0;i < datastack2.size();i++)
		{
			result.push_back( (datastack2.top() )->val);
			datastack2.pop();
		}

datastack2每pop出一次数据，datastack2.size()就会减一，同时i++，导致过早的结束了这个循环，正确的写法如下:

		int length = datastack2.size();
		for (int i = 0; i < length; i++)
		{
			result.push_back((datastack2.top())->val);
			datastack2.pop();
		}