二叉树层序遍历的递归和非递归算法

层序遍历，顾名思义，就是一层一层的遍历，在二叉树中就是从根节点到叶子结点一层层遍历，递归算法比较好理解，非递归算法需要用到队列，先将根节点入队列，输出其节点值，有左右子节点，依次将左右子节点入队列，先入队列的先输出，依此类推。代码实现如下：

#include<iostream>#include<vector>  #include<stack>  #include<queue> using namespace std;struct TreeNode {int val;TreeNode *left;TreeNode *right;TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}};class Solution1 {public://递归遍历(Recursive Solution)static vector<vector<int>> levelOrder(TreeNode *root) {vector<vector<int>> res;levelorder(root, 0, res);return res;}static void levelorder(TreeNode *root, int level, vector<vector<int>> &res) {if (!root) return;if (res.size() == level) res.push_back({});res[level].push_back(root->val);if (root->left) levelorder(root->left, level + 1, res);if (root->right) levelorder(root->right, level + 1, res);}};class Solution2 {public://迭代遍历(Iterative Solution)static vector<vector<int>> levelOrder(TreeNode *root) {vector<vector<int>> res;if(!root) return res;queue<TreeNode*> q;q.push(root);while(!q.empty()){int n=q.size();vector<int> onelevel;for(int i=0;i<n;i++){TreeNode *node=q.front();onelevel.push_back(node->val);q.pop();if(node->left) q.push(node->left);if(node->right) q.push(node->right);}res.push_back(onelevel);}return res;}};int main() {TreeNode * root = NULL;root = new TreeNode(0);root->left = new TreeNode(1);root->right = new TreeNode(2);root->left->left = new TreeNode(3);root->left->right = new TreeNode(4);root->right->left = new TreeNode(5);root->right->right = new TreeNode(6);vector<vector<int>> res = Solution1::levelOrder(root);for (int i = 0; i<res.size(); i++) {for (int j = 0; j<res[i].size(); j++) {cout << res[i][j] << ' ';}cout << endl;}cout<<endl;vector<vector<int>> result = Solution2::levelOrder(root);for (int i = 0; i<result.size(); i++) {for (int j = 0; j<result[i].size(); j++) {cout << result[i][j] << ' ';}cout << endl;}//cout<<endl;system("pause");return 0;}