Algorithms—107.Binary Tree Level Order Traversal II

/**
* Definition for a binary tree node.
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public List<List<Integer>> levelOrderBottom(TreeNode root) {
List<List<TreeNode>> answerList = new ArrayList<List<TreeNode>>();
List<List<Integer>> list = new ArrayList<List<Integer>>();
if (root == null) {
return list;
}
List<TreeNode> initList = new ArrayList<TreeNode>();
initList.add(root);
answerList.add(initList);
for (int i = 0; i < answerList.size(); i++) {
List<TreeNode> newList = new ArrayList<TreeNode>();
for (int j = 0; j < answerList.get(i).size(); j++) {
TreeNode tree = answerList.get(i).get(j);
if (tree.left != null) {
newList.add(tree.left);
}
if (tree.right != null) {
newList.add(tree.right);
}
}
if (newList.size() != 0) {
answerList.add(newList);
}
}
for (int i = answerList.size()-1; i >=0; i--) {
List<Integer> l = new ArrayList<Integer>();
for (int j = 0; j < answerList.get(i).size(); j++) {
l.add(answerList.get(i).get(j).val);
}
list.add(l);
}

return list;
}
}