广度优先搜索
- 思路：
- 广度优先搜索对应的数据结构是队列，处理每一层时把下一层节点 push 到队列中；
- 使用 vector<vector> 数据结构存放结果；
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode() : val(0), left(nullptr), right(nullptr) {}
* TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
* TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
* };
*/
class Solution {
public:
vector<vector<int>> levelOrder(TreeNode* root) {
std::vector<std::vector<int>> result;
if (root == nullptr) {
return result;
}
std::queue<TreeNode*> q;
q.push(root);
while (!q.empty()) {
int levelSize = q.size();
result.push_back(std::vector<int>());
for (int idx = 0; idx < levelSize; ++idx) {
auto node = q.front();
q.pop();
result.back().push_back(node->val);
if (node->left) {
q.push(node->left);
}
if (node->right) {
q.push(node->right);
}
}
}
return result;
}
};
