二叉树的层序遍历

#include <iostream>
#include <queue>
#include <vector>

using namespace std;

struct TreeNode {
    int val;
    TreeNode* left;
    TreeNode* right;
    TreeNode() : val(0), left(nullptr), right(nullptr) {}
    TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
    TreeNode(int x, TreeNode* left, TreeNode* right) : val(x), left(left), right(right) {}
};

class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        queue<TreeNode*> que;
        vector<vector<int>> res;
        // 将树节点添加到队列
        if (root != nullptr)
            que.push(root);
        while (!que.empty()) {
            vector<int> tmp;
            for (int i = que.size(); i > 0; --i) {
                root = que.front();
                // 出队,方便后边添加左右节点到队列
                que.pop();
                tmp.push_back(root->val);
                if (root->left != nullptr)
                    que.push(root->left);
                if (root->right != nullptr)
                    que.push(root->right);
            }
            res.push_back(tmp);
        }
        return res;
    }
};

int main() {
    TreeNode* root =  new TreeNode(3);
    TreeNode* TreeNode1 =  new TreeNode(9);
    TreeNode* TreeNode2 =  new TreeNode(20);
    TreeNode* TreeNode3 =  new TreeNode(15);
    TreeNode* TreeNode4 =  new TreeNode(7);
    root->left = TreeNode1;
    root->right = TreeNode2;
    TreeNode1->left = nullptr;
    TreeNode1->right = nullptr;
    TreeNode2->left = TreeNode3;
    TreeNode2->right = TreeNode4;
    TreeNode3->left = nullptr;
    TreeNode3->right = nullptr;
    TreeNode4->left = nullptr;
    TreeNode4->right = nullptr;
    Solution solution;
    vector<vector<int>> result = solution.levelOrder(root);
    for (int i = 0; i < result.size(); ++i) {
        cout << "[";
        for (int j = 0; j < result[i].size(); ++j) {
            cout <<"[" <<result[i][j] << "]";
        }
        cout << "]" << endl;
    }
}