Leetcode:find Leaves of Binary Tree

 Given A binary tree, collect a tree ' s nodes as if you were doing this:collect and remove all leaves, repeat until th E Tree is empty. example:given binary tree  1/ /span>2 3/ \  4 5 Returns [ 4, 5, 3], [2], [1 1. Removing the leaves [4, 5, 3] would result in this   tree :  1/2 2.          Now removing the leaf [2] would result in this   tree:  1 3. Now removing the leaf [14, 5, 3], [2], [1]. 

Better SOLUTION:HTTPS://DISCUSS.LEETCODE.COM/TOPIC/49194/10-LINES-SIMPLE-JAVA-SOLUTION-USING-RECURSION-WITH-EXPLANATION/2

For the question we need to take bottom-up approach. The key is to find the height of each node. The height of a node is the number of edges from the node to the deepest leaf.

1 /**2 * Definition for a binary tree node.3 * public class TreeNode {4 * int val;5 * TreeNode left;6 * TreeNode right;7 * TreeNode (int x) {val = x;}8  * }9  */Ten  Public classSolution { One      PublicList<list<integer>>findleaves (TreeNode root) { Alist<list<integer>> res =NewArraylist<>(); - Helper (root, res); -         returnRes; the     } -      -      Public intHelper (TreeNode cur, list<list<integer>>Res) { -         if(cur = =NULL)return-1; +         intLevel = 1 +Math.max (Helper (Cur.left, res), helper (Cur.right, res)); -         if(Res.size () <=Level ) +Res.add (NewArraylist<integer>()); A Res.get (Level). Add (cur.val); atCur.left = Cur.right =NULL; -         returnLevel ; -     } -}

First time solution:hashset+ DFS

1 /**2 * Definition for a binary tree node.3 * public class TreeNode {4 * int val;5 * TreeNode left;6 * TreeNode right;7 * TreeNode (int x) {val = x;}8  * }9  */Ten  Public classSolution { One      PublicList<list<integer>>findleaves (TreeNode root) { Aarraylist<list<integer>> res =NewArraylist<list<integer>>(); -         if(Root = =NULL)returnRes; -Hashset<treenode> visited =NewHashset<>(); the          while(!visited.contains (Root)) { -Arraylist<integer> leaves =NewArraylist<integer>(); - helper (root, leaves, visited); -Res.add (NewArraylist<integer>(leaves)); +         } -         returnRes; +     } A      at      Public voidHelper (TreeNode cur, arraylist<integer> leaves, hashset<treenode>visited) { -         if((cur.left==NULL|| Visited.contains (Cur.left)) && (cur.right==NULL||visited.contains (cur.right))) { - Leaves.add (cur.val); - visited.add (cur); -             return; -         } in         if(cur.left!=NULL&&!visited.contains (cur.left)) - Helper (Cur.left, leaves, visited); to         if(cur.right!=NULL&&!visited.contains (cur.right)) + Helper (Cur.right, leaves, visited); -     } the}

Leetcode:find Leaves of Binary Tree