LeetCode-M-Binary Tree Level Order Traversal

題意

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],

    3
   / \
  9  20
    /  \
   15   7

return its level order traversal as:

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

解法

利用隊列，層次遍歷

實現

/**
 * 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>> result;
        if(root == NULL) return result;
        queue<TreeNode*> tq;
        tq.push(root);
        tq.push(NULL);
        while(!tq.empty() && tq.front() != NULL){
            vector<int> sr;
            while(tq.front() != NULL){
                TreeNode* node = tq.front();
                tq.pop();
                sr.push_back(node->val);
                if(node->left != NULL) tq.push(node->left);
                if(node->right != NULL) tq.push(node->right);
            }
            tq.push(NULL);
            result.push_back(sr);
            tq.pop();
        }
        return result;
    }
};