题解 | #求二叉树的层序遍历#

/**
 * struct TreeNode {
 *    int val;
 *    struct TreeNode *left;
 *    struct TreeNode *right;
 * };
 */

class Solution {
public:
    /**
     * 
     * @param root TreeNode类 
     * @return int整型vector<vector<>>
     */
    vector<vector<int> > levelOrder(TreeNode* root) {
        if (root == nullptr) return {};

        queue<TreeNode*> q;
        q.push(root);
        vector<vector<int> > res;

        while(!q.empty()) {
            res.push_back(vector<int>()); // 装了一层的遍历值
            int size = q.size(); // 注意这里的size计算要在for之前计算好
            for (int i = 0; i < size; i++) {
                TreeNode* node = q.front();
                q.pop();
                if(node) res.back().push_back(node->val);
                if(node->left) q.push(node->left);
                if(node->right) q.push(node->right);
            }
        }
        return res;
        // write code here
    }
};