层序遍历二叉树

/*
struct TreeNode {
    int val;
    struct TreeNode *left;
    struct TreeNode *right;
    TreeNode(int x) :
            val(x), left(NULL), right(NULL) {
    }
};*/
class Solution {
public:
    vector<int> PrintFromTopToBottom(TreeNode* root) {
        vector<int> v;
        if(root == NULL){
            //没有该判断，会出现段错误
            return v;
        }
        queue<TreeNode*> q;
        q.push(root);
        while(!q.empty()){
            TreeNode* temp;
            temp = q.front();
            q.pop();
            v.push_back(temp->val);
            if(temp->left != NULL){
                q.push(temp->left);
            }
            if(temp->right != NULL){
                q.push(temp->right);
            }
        }
        return v;
    }
};