[GeeksForGeeks] Generate all root to leaf paths of a binary tree

Given a binary tree, generate all root to leaf paths of a binary tree.

Example:

Example Tree

The output for the above example is [[1, 2, 4], [1, 2, 5], [1,3]]

Key idea: root to leaf path, root node comes first, so use pre-order traversal.

import java.util.ArrayList;

public class RootToLeafPaths {
    public static ArrayList<ArrayList<Integer>> getRootToLeafPaths(TreeNode root) {
        ArrayList<ArrayList<Integer>> paths = new ArrayList<>();
        if(root == null) {
            return paths;
        }
        preOrder(paths, new ArrayList<Integer>(), root);
        return paths;
    }
    private static void preOrder(ArrayList<ArrayList<Integer>> paths, ArrayList<Integer> path, TreeNode node) {
        path.add(node.val);
        if(node.left == null && node.right == null) {
            paths.add(new ArrayList<Integer>(path));
        }
        else {
            preOrder(paths, path, node.left);
            preOrder(paths, path, node.right);            
        }
        path.remove(path.size() - 1);
    }
    public static void main(String[] args) {
        TreeNode[] nodes = new TreeNode[5];
        for(int i = 0; i < 5; i ++) {
            nodes[i] = new TreeNode(i);
        }
        nodes[0].left = nodes[1]; nodes[0].right = nodes[2];
        nodes[1].left = nodes[3]; nodes[1].right = nodes[4];
        ArrayList<ArrayList<Integer>> paths = getRootToLeafPaths(nodes[0]);
        for(int i = 0; i < paths.size(); i++) {
            for(int j = 0; j < paths.get(i).size(); j++) {
                System.out.print(paths.get(i).get(j) + " ");
            }
            System.out.println();
        }        
    }
}