【Tree】二叉树先序遍历 迭代 & 递归

  1 /***************************
  2 https://leetcode.com/problems/binary-tree-preorder-traversal/
  3 @date 2015.5.13
  4 @description
  5 用非递归方法对二叉树进行先序遍历
  6 借助辅助栈
  7 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。
  8
  9
 10 ****************************/
 11
 12 #include <iostream>
 13 #include <stack>
 14 #include <vector>
 15
 16 using namespace std;
 17
 18 struct TreeNode{
 19     int val;
 20     TreeNode *left, *right;
 21     TreeNode(int x): val(x), left(NULL), right(NULL) {}
 22 };
 23
 24 class Solution{
 25 public:
 26     vector<int> preorderTraversal(TreeNode* root) {
 27         stack<TreeNode *> s;
 28         vector<int> res;
 29         if (!root)
 30             return res;
 31
 32         while (root != NULL || !s.empty()){
 33             while (root != NULL){
 34                 res.push_back(root->val);
 35                 s.push(root);
 36                 root = root->left;
 37             }
 38
 39             if (!s.empty()){
 40                 root = s.top();
 41                 s.pop();
 42                 root = root->right;
 43             }
 44         }
 45         return res;
 46     }
 47 };
 48
 49
 50 // 第二种方法迭代先序遍历二叉树
 51 vector<int> preorderTraversal(TreeNode *root){
 52     vector<int> res;
 53     if (!root)
 54         return res;
 55     stack<TreeNode *> s;
 56     s.push(root);
 57     while (!s.empty()){
 58         TreeNode *temp = s.top();
 59         s.pop();
 60         res.push_back(temp->val);
 61         if (temp->right) s.push(temp->right); // 先对右孩子入栈
 62         if (temp->left) s.push(temp->left);
 63     }
 64     return res;
 65 }
 66
 67 TreeNode *insert(TreeNode *root, int data){
 68     TreeNode *ptr = root;
 69     TreeNode *tempNode; // 存储的是插入节点的父节点
 70     TreeNode *newNode = new TreeNode(data);
 71
 72     if (ptr == NULL)
 73         return newNode;
 74     else{
 75         while (ptr != NULL){
 76             tempNode = ptr;
 77             if (ptr->val >= data){
 78                 ptr = ptr->left;
 79             }else{
 80                 ptr = ptr->right;
 81             }
 82         }
 83         if (tempNode->val >= data){
 84             tempNode->left = newNode;
 85         }else{
 86             tempNode->right = newNode;
 87         }
 88     }
 89     return root;
 90 }
 91
 92 // 递归先序遍历二叉树
 93 void travPre(TreeNode *root){
 94     if (!root) return;
 95     cout << root->val << " ";
 96     travPre(root->left);
 97     travPre(root->right);
 98 }
 99
100
101 int main(){
102     TreeNode *root = NULL;
103     int temp = 0;
104     cin >> temp;
105     while (temp != 0){ // 以0结尾（输入0终止）
106         root = insert(root, temp);
107         cin >> temp;
108     } // 创建一棵二叉树
109
110     // 递归先序遍历
111   //  travPre(root);
112     Solution a;
113     vector<int> res = preorderTraversal(root);
114     for (vector<int>::iterator it = res.begin(); it != res.end(); ++it)
115         cout << *it << " ";
116
117 }