题解 | #求二叉树的层序遍历#

利用队列先进先出的特性，分别将二叉树各个层的结点保存进入队列，使用变量size保存每次队列中元素个数，以保证每一次进入循环后都可将队列清空，即每个结点都被遍历到。

/**
 * 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) {
        // write code here
        vector<vector<int>> ans;
        vector<int> res;
        if (root == NULL)
            return ans;
        queue<TreeNode* > q;
        q.push(root);
        while (!q.empty()) {
            TreeNode* t;
            res.clear();
            int size = q.size();
            while (size--) {
                t = q.front(); q.pop();
                res.push_back(t->val);
                if (t->left)
                    q.push(t->left);
                if (t->right)
                    q.push(t->right);
            }
            ans.push_back(res);
        }
        return ans;
    }
};