题解 | 求二叉树的层序遍历

/**
 * struct TreeNode {
 *  int val;
 *  struct TreeNode *left;
 *  struct TreeNode *right;
 *  TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 * };
 */
class Solution {
  public:
    vector< vector<int> > ans;
    vector< int > temp;

    vector<vector<int> > levelOrder(TreeNode* root) {
        if(!root){
            return {};
        }

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

        int size = 1;
        int new_size = 0;
        while (!q.empty()) {  //因为结果需要二维向量，每一层一个向量，使用了双层循环，内层添加一层的值，外层进行添加进ans，更新下一层的节点数，清空temp准备记录下一层；
            while ( size > 0 ) {
                TreeNode* node = q.front();q.pop();
                temp.push_back(node->val);
                if (node->left) {
                    q.push(node->left);
                    new_size++;
                }
                if (node->right) {
                    q.push(node->right);
                    new_size++;
                }
                size--;
            }
            size = new_size;
            new_size = 0;

            ans.push_back(temp);
            temp.clear();
        }

        return ans;
    }
};