层序遍历

/**
 * struct TreeNode {
 *    int val;
 *    struct TreeNode *left;
 *    struct TreeNode *right;
 * };
 */

class Solution {
public:
    /**
     * 
     * @param root TreeNode类 
     * @return int整型vector<vector<>>
     */
    vector<vector<int> > levelOrder(TreeNode* root) {
        vector<vector<int>> result;
        queue<TreeNode*> levelNode;
        TreeNode* currentNode;
        if (root == nullptr)
            return result;
        vector<int> curLevel;
        levelNode.push(root);
        while (!levelNode.empty())
        {
            curLevel.clear();
            int n = levelNode.size();
            for (int i = 0; i < n; i++)
            {
                currentNode = levelNode.front();
                levelNode.pop();
                curLevel.push_back(currentNode -> val);
                if (currentNode -> left != nullptr)
                    levelNode.push(currentNode -> left);
                if (currentNode -> right != nullptr)
                    levelNode.push(currentNode -> right);
            }
            result.push_back(curLevel);
        }
        return result;
    }
};