【Tree】二叉树先序遍历 迭代 & 递归

/***************************
https://leetcode.com/problems/binary-tree-preorder-traversal/
@date 2015.5.13
@description
用非递归方法对二叉树进行先序遍历
借助辅助栈
每次先访问根节点，把节点压入栈，再转向其左孩子，直至左子树的左孩子为空，依次将栈顶元素出栈，转向右孩子。


****************************/

#include <iostream>
#include <stack>
#include <vector>

using namespace std;

struct TreeNode{
    int val;
    TreeNode *left, *right;
    TreeNode(int x): val(x), left(NULL), right(NULL) {}
};

class Solution{
public:
    vector<int> preorderTraversal(TreeNode* root) {
        stack<TreeNode *> s;
        vector<int> res;
        if (!root)
            return res;

        while (root != NULL || !s.empty()){
            while (root != NULL){
                res.push_back(root->val);
                s.push(root);
                root = root->left;
            }

            if (!s.empty()){
                root = s.top();
                s.pop();
                root = root->right;
            }
        }
        return res;
    }
};


// 第二种方法迭代先序遍历二叉树
vector<int> preorderTraversal(TreeNode *root){
    vector<int> res;
    if (!root)
        return res;
    stack<TreeNode *> s;
    s.push(root);
    while (!s.empty()){
        TreeNode *temp = s.top();
        s.pop();
        res.push_back(temp->val);
        if (temp->right) s.push(temp->right); // 先对右孩子入栈
        if (temp->left) s.push(temp->left);
    }
    return res;
}

TreeNode *insert(TreeNode *root, int data){
    TreeNode *ptr = root;
    TreeNode *tempNode; // 存储的是插入节点的父节点
    TreeNode *newNode = new TreeNode(data);

    if (ptr == NULL)
        return newNode;
    else{
        while (ptr != NULL){
            tempNode = ptr;
            if (ptr->val >= data){
                ptr = ptr->left;
            }else{
                ptr = ptr->right;
            }
        }
        if (tempNode->val >= data){
            tempNode->left = newNode;
        }else{
            tempNode->right = newNode;
        }
    }
    return root;
}

// 递归先序遍历二叉树
void travPre(TreeNode *root){
    if (!root) return;
    cout << root->val << " ";
    travPre(root->left);
    travPre(root->right);
}


int main(){
    TreeNode *root = NULL;
    int temp = 0;
    cin >> temp;
    while (temp != 0){ // 以0结尾（输入0终止）
        root = insert(root, temp);
        cin >> temp;
    } // 创建一棵二叉树

    // 递归先序遍历
  //  travPre(root);
    Solution a;
    vector<int> res = preorderTraversal(root);
    for (vector<int>::iterator it = res.begin(); it != res.end(); ++it)
        cout << *it << " ";

}