剑指 Offer 32 - I. 从上到下打印二叉树

二叉树的层序遍历

/**
 * 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<int> levelOrder(TreeNode* root) {
        if(root == nullptr) return {};
        vector<int> ans; ans.clear();
        queue<TreeNode*> Q;
        while(!Q.empty()) {
            Q.pop();
        }

        Q.push(root);
        while(!Q.empty()) {
            TreeNode* h = Q.front();
            Q.pop();
            ans.push_back(h->val);
            if(h->left) Q.push(h->left);
            if(h->right) Q.push(h->right);
        }
        return ans;
    }
};