剑指 Offer 32

BFS。

/**
 * 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) return {};
        vector<int> res;
        queue<TreeNode*> q;
        q.push(root);
        while (q.size()) {
            TreeNode* t = q.front();
            q.pop();

            res.push_back(t->val);
            if (t->left) q.push(t->left);
            if (t->right) q.push(t->right);
        }
        return res;
    }
};