LEETCODE ALGORITHM:102. Binary Tree Level Order Traversal

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题目

Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).

For example: Given binary tree [3,9,20,null,null,15,7],

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2
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5
  3
 / \
9  20
  /  \
 15   7

return its level order traversal as:

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2
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5
[
  [3],
  [9,20],
  [15,7]
]

题解

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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<pair<TreeNode*,int>> q;
        vector<vector<int>> ans;
        if(root==NULL) return ans;
        vector<int> tmp;
        int l=0;
        q.push({root,0});
        while(!q.empty())
        {
            auto [t1,t2]=q.front();
            q.pop();
            if(t2==l)
            {
                tmp.push_back(t1->val);
            }else{
                ans.push_back(tmp);
                tmp.clear();
                tmp.push_back(t1->val);
                l=t2;
            }
            if(t1->left) q.push({t1->left,t2+1});
            if(t1->right) q.push({t1->right,t2+1});
        }
        ans.push_back(tmp);
        return ans;
    }
};