SumRootToLeafNumbers.java

package medium;

import java.util.ArrayList;
import java.util.List;

import classes.TreeNode;

public class SumRootToLeafNumbers {
    //it could overflow when the tree is very deep/large, so definitely use long
    //let's assume it won't overflow first
    public int sumNumbers(TreeNode root) {
        if(root == null) return 0;
        List<Integer> allNumbers = new ArrayList();
        dfs(root, new StringBuilder(), allNumbers);
        int sum = 0;
        for(int i : allNumbers){
            sum += i;
        }
        return sum;
    }

    private void dfs(TreeNode root, StringBuilder sb, List<Integer> allNumbers) {
        sb.append(root.val);
        if(root.left != null){
            dfs(root.left, sb, allNumbers);
        }
        if(root.right != null){
            dfs(root.right, sb, allNumbers);
        }
        if(root.left == null && root.right == null){
            allNumbers.add(Integer.parseInt(sb.toString()));
        }
        sb.deleteCharAt(sb.length()-1);
    }
    
    public static void main(String... strings) {
        more_concise_version test = new more_concise_version();
        TreeNode root = new TreeNode(1);
        root.left = new TreeNode(2);
        root.right = new TreeNode(3);
        System.out.println(test.sumNumbers(root));
    }
}

class more_concise_version{
    public int sumNumbers(TreeNode root) {
        return dfs(root, 0);
    }

    private int dfs(TreeNode root, int sum) {
        if(root == null) return 0;
        if(root.left == null && root.right == null) return sum*10+root.val;
        return dfs(root.left, sum*10+root.val) + dfs(root.right, sum*10+root.val);
    }
}