366. Find Leaves of Binary Tree

Given the root of a binary tree, collect a tree's nodes as if you were doing this:

• Collect all the leaf nodes.

• Remove all the leaf nodes.

• Repeat until the tree is empty.

Example 1:

Input: root = [1,2,3,4,5]
Output: [[4,5,3],[2],[1]]
Explanation:
[[3,5,4],[2],[1]] and [[3,4,5],[2],[1]] are also considered correct answers since per each level it does not matter the order on which elements are returned.

Example 2:

Input: root = [1]
Output: [[1]]

Constraints:

• The number of nodes in the tree is in the range [1, 100].

• -100 <= Node.val <= 100

Solution

Build the materials that we need for topological sorting. - Graph and degree

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<List<Integer>> findLeaves(TreeNode root) {
        Map<TreeNode, Integer> degree = new HashMap<>();
        Map<TreeNode, TreeNode> parents = new HashMap<>();
        buildGraph(root, degree, parents);
        Queue<TreeNode> queue = new LinkedList<>();
        for(TreeNode node : degree.keySet()){
            if(degree.get(node) == 0){
                queue.add(node);
            }
        }
        List<List<Integer>> ret = new ArrayList<>();
        while(!queue.isEmpty()){
            int count = queue.size();
            List<Integer> temp = new LinkedList<>();
            while(count > 0){
                TreeNode node = queue.poll();
                temp.add(node.val);
                TreeNode parent = parents.get(node);
                if(parent != null) {
                    degree.put(parent, degree.get(parent)-1);
                    if(degree.get(parent) == 0){
                        queue.add(parent);
                    }
                }
                count--;
            }
            ret.add(temp);
        }
        return ret;
        
    }
    
    
    public void buildGraph(TreeNode node, Map<TreeNode,Integer> degree, Map<TreeNode, TreeNode> parents){
        if(node == null) return;
        int count = 0;
        if(node.left != null){
            parents.put(node.left, node);
            buildGraph(node.left, degree, parents);
            count++;
        }
        if(node.right != null){
            parents.put(node.right, node);
            buildGraph(node.right, degree, parents);
            count++;
        }
        degree.put(node, count);
    }
}