leetcode-二叉樹的層序遍歷

題目

https://leetcode-cn.com/problems/binary-tree-level-order-traversal/

思路

隊列實現BFS；使用隊列來存放每層的節點，從第一層開始檢索，如果有孩子就壓入。隊列：先進先出，遍歷隊列的front，即可實現BFS

AC代碼

/**
 * 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;
        queue<TreeNode *>q;
        if(root == NULL)    return res;
        q.push(root);
        while (!q.empty()){
            vector<int>temp;
            int len = q.size();
            for(int i = 0;i < len;i++){ 
                //注意這個len不能直接寫成q.size()，因爲後面有新的插入，會重新計算q.size();
                TreeNode * node = q.front();
                q.pop();
                temp.push_back(node->val);
                if(node->left)  q.push(node->left);
                if(node->right) q.push(node->right);
            }
            res.push_back(temp);
        }
        return res;
        
    }
};