LeetCode-102 二叉树的层序遍历

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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) {
        vector<vector<int>> res;
        if(root == nullptr)
            return res;
        queue<pair<TreeNode*, int>> q;
        q.push(make_pair(root, 0));
        while(!q.empty())
        {
            TreeNode *node = q.front().first;
            int level = q.front().second;
            q.pop();
            if(level == res.size())
                res.push_back(vector<int>());
            res[level].push_back(node->val);
            
            if(node->left != nullptr)
                q.push(make_pair(node->left, level + 1));
            if(node->right != nullptr)
                q.push(make_pair(node->right, level + 1));
        }
        return res;
    }
};