Binary tree traversal non-recursive program -- using stack to simulate system stack

一、前序遍历

给定一个二叉树,返回它的 前序 遍历.

示例:

输入: [1,null,2,3]     1    \     2    /   3 输出: [1,2,3]

进阶: 递归算法很简单,你可以通过迭代算法完成吗？

前序遍历C++代码

/** * Definition for a binary tree node. * struct TreeNode { * int val; * TreeNode *left; * TreeNode *right; * TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */struct Command{    string s; // go, print    TreeNode* node;    Command(string s, TreeNode* node): s(s), node(node){}};class Solution {public:    vector<int> preorderTraversal(TreeNode* root) {        vector<int> res;        if (root == NULL)        {            return res;        }                stack<Command> stack;        stack.push(Command("go", root));        while ( !stack.empty() )        {            Command command = stack.top();            stack.pop();            if (command.s == "print")            {                res.push_back(command.node -> val);            }            else            {                assert( command.s == "go");                if (command.node -> right)                {                    stack.push(Command("go", command.node -> right));                }                if (command.node -> left)                {                    stack.push(Command("go", command.node -> left));                }                stack.push(Command("print", command.node));            }         }        return res;    }};

二、中序遍历

给定一个二叉树,返回它的中序 遍历.

示例:

输入: [1,null,2,3]   1    \     2    /   3输出: [1,3,2]

进阶: 递归算法很简单,你可以通过迭代算法完成吗？

中序遍历C++代码

// @lc code=start/** * Definition for a binary tree node. * struct TreeNode { * int val; * TreeNode *left; * TreeNode *right; * TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */struct Command{    string s; // go, print    TreeNode *node;    Command(string s, TreeNode *node) : s(s), node(node) {}};class Solution{public:    vector<int> inorderTraversal(TreeNode *root)    {        vector<int> res;        if (root == NULL)        {            return res;        }        stack<Command> stack;        stack.push(Command("go", root));        while (!stack.empty())        {            Command command = stack.top();            stack.pop();            if (command.s == "print")            {                res.push_back(command.node->val);            }            else            {                assert(command.s == "go");                                if (command.node->right)                {                    stack.push(Command("go", command.node->right));                }                stack.push(Command("print", command.node));                if (command.node->left)                {                    stack.push(Command("go", command.node->left));                }            }        }        return res;    }};// @lc code=end

三、后序遍历

给定一个二叉树,返回它的 后序 遍历.

示例:

输入: [1,null,2,3]     1    \     2    /   3 输出: [3,2,1]

进阶: 递归算法很简单,你可以通过迭代算法完成吗？

后序遍历C++代码

// @lc code=start/** * Definition for a binary tree node. * struct TreeNode { * int val; * TreeNode *left; * TreeNode *right; * TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */struct Command{    string s; // go, print    TreeNode *node;    Command(string s, TreeNode *node) : s(s), node(node) {}};class Solution{public:    vector<int> postorderTraversal(TreeNode *root)    {        vector<int> res;        if (root == NULL)        {            return res;        }        stack<Command> stack;        stack.push(Command("go", root));        while (!stack.empty())        {            Command command = stack.top();            stack.pop();            if (command.s == "print")            {                res.push_back(command.node->val);            }            else            {                assert(command.s == "go");                stack.push(Command("print", command.node));                if (command.node->right)                {                    stack.push(Command("go", command.node->right));                }                if (command.node->left)                {                    stack.push(Command("go", command.node->left));                }            }        }        return res;    }};// @lc code=end

练习题： 341.Flatten Nested List Iterator