Solution

• 层序遍历

/*
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> res;
        if (root == nullptr) return res;
        
        queue<TreeNode *> que;
        TreeNode *cur = root;
        
        que.push(cur);
        res.push_back(cur->val);
        while (!que.empty()) {
            int n = que.size();
            for (int i = 0; i < n; ++i) {
                cur = que.front();
                que.pop();
                if (cur->left) {
                    res.push_back(cur->left->val);
                    que.push(cur->left);
                }
                if (cur->right) {
                    res.push_back(cur->right->val);
                    que.push(cur->right);
                }
            }
        }
        return res;
    }
};