Binary Tree Preorder Traversal

非递归用栈实现先序遍历

#include <iostream>#include <vector>#include <stack> using namespace std;struct TreeNode {int val;TreeNode *left;TreeNode *right;    TreeNode(int x) : val(x), left(NULL), right(NULL) {}};stack<TreeNode*> st;class Solution {public:    vector<int> preorderTraversal(TreeNode *root) {    vector<int> v;    TreeNode *p;    while(root || !st.empty())        {        while(root)        {        v.push_back(root->val);        st.push(root);        root = root->left;        }        if(!st.empty())        {        root = st.top();st.pop();        }        root = root->right;        }        return v;    }};int main(void){Solution s;/*TreeNode *root = new TreeNode(1);TreeNode *node1 = new TreeNode(2);  root->right = node1;TreeNode *node2 = new TreeNode(3);  node1->left = node2;*//*TreeNode *root = new TreeNode(1);TreeNode *node1 = new TreeNode(4);  root->left = node1;TreeNode *node2 = new TreeNode(3);  root->right = node2;TreeNode *node3 = new TreeNode(2);  node1->left = node3;*//*TreeNode *root = new TreeNode(3);TreeNode *node2 = new TreeNode(1);  root->right = node2;TreeNode *node3 = new TreeNode(2);  node2->left = node3;*/TreeNode *root = new TreeNode(2);TreeNode *node1 = new TreeNode(1);  root->left = node1;TreeNode *node2 = new TreeNode(3);  root->right = node2;TreeNode *node3 = new TreeNode(4);  node1->right = node3;vector<int> re = s.preorderTraversal(root);vector<int>::iterator iter;for(iter = re.begin(); iter != re.end(); ++iter){cout << *iter << endl;}return 0;}