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July2_2020.java
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July2_2020.java
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// Binary Tree Level Order Reverse Traversal
class Solution {
public List<List<Integer>> levelOrderBottom(TreeNode root) {
List<List<Integer>> result = new ArrayList<List<Integer>>();
if(root == null){
return result;
}
LinkedList<TreeNode> current = new LinkedList<TreeNode>();
LinkedList<TreeNode> next = new LinkedList<TreeNode>();
current.offer(root);
ArrayList<Integer> numberList = new ArrayList<Integer>();
// need to track when each level starts
while(!current.isEmpty()){
TreeNode head = current.poll();
numberList.add(head.val);
if(head.left != null){
next.offer(head.left);
}
if(head.right!= null){
next.offer(head.right);
}
if(current.isEmpty()){
current = next;
next = new LinkedList<TreeNode>();
result.add(numberList);
numberList = new ArrayList<Integer>();
}
}
//return Collections.reverse(result);
List<List<Integer>> reversedResult = new ArrayList<List<Integer>>();
for(int i=result.size()-1; i>=0; i--){
reversedResult.add(result.get(i));
}
return reversedResult;
}
}