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102.二叉树的层序遍历

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1. queue实现层次遍历

时间O(n)，空间O(n)。 

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/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        queue<TreeNode*> q;
        vector<vector<int>> ans;

        if (root != nullptr)
            q.push(root);
        while (!q.empty()) {
            vector<int> sub;
            int qsz = q.size();
            while (qsz--) {
                TreeNode* temp = q.front();
                q.pop();
                sub.push_back(temp->val);
                if (temp->left != nullptr)
                    q.push(temp->left);
                if (temp->right != nullptr)
                    q.push(temp->right);
            }
            ans.push_back(sub);
        }

        return ans;
    }
};