Given a binary tree, return the bottom-up level order traversal of its nodes' values. (ie, from left to right, level by level from leaf to root).
For example:
Given binary tree [3,9,20,null,null,15,7],
3
/ \
9 20
/ \
15 7
return its bottom-up level order traversal as:
[
[15,7],
[9,20],
[3]
]
与之前题目相同，依次遍历左节点、右节点，最后颠倒顺序即可。
class Solution {
public:
vector<vector<int>> levelOrderBottom(TreeNode* root) {
vector<vector<int>> res;
lo(root, 0, res);
reverse(res.begin(), res.end());
return res;
}
void lo(TreeNode *root, int level, vector<vector<int>>& res){
if(!root) return;
if(res.size()==level){
res.push_back({});
}
res[level].push_back(root->val);
lo(root->left, level+1, res);
lo(root->right, level+1, res);
}
};
