/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
解法一：迭代
class Solution {
public:
vector<vector<int>> levelOrderBottom(TreeNode* root) {
vector<vector<int>> result;
if(root==nullptr)
return result;
queue<TreeNode*> current,next;
vector<int> level;
current.push(root);
while(!current.empty())
{
while(!current.empty())
{
TreeNode* node=current.front();
current.pop();
level.push_back(node->val);
if(node->left!=nullptr)
next.push(node->left);
if(node->right!=nullptr)
next.push(node->right);
}
result.push_back(level);
level.clear();
swap(next,current);
}
reverse(result.begin(),result.end());
return result;
}
};
解法二：递归
class Solution {
public:
vector<vector<int>> levelOrderBottom(TreeNode* root) {
vector<vector<int>> result;
traverse(root,1,result);
reverse(result.begin(),result.end());
return result;
}
void traverse(TreeNode* root,size_t level,vector<vector<int>> &result)
{
if(!root)
return;
if(level>result.size())
result.push_back(vector<int>());
result[level-1].push_back(root->val);
traverse(root->left,level+1,result);
traverse(root->right,level+1,result);
}
};