102. Binary Tree Level Order Traversal

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]
]

问题比较明显，就是按层数从左往右输出每个TreeNode节点的值就可以了。很容易想到广度优先遍历。

class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        vector<vector<int>> res;
        queue<TreeNode*> q;
        if (root) {
            q.push(root);
            while (q.size()) {
                vector<int> temp;
                int len = q.size();
                for (int i=0; i<len; ++i) {
                    temp.push_back(q.front()->val);
                    if (q.front()->left)    q.push(q.front()->left);
                    if (q.front()->right)   q.push(q.front()->right);
                    q.pop();
                }
                res.push_back(temp);
            }
        }
        return res;
    }
};