LeetCode 1022 - Sum of Root To Leaf Binary Numbers

https://leetcode.com/problems/sum-of-root-to-leaf-binary-numbers/

Given a binary tree, each node has value 0 or 1.  Each root-to-leaf path represents a binary number starting with the most significant bit.  For example, if the path is 0 -> 1 -> 1 -> 0 -> 1, then this could represent 01101 in binary, which is 13.

For all leaves in the tree, consider the numbers represented by the path from the root to that leaf.

Return the sum of these numbers.

Example 1:

Input: [1,0,1,0,1,0,1]
Output: 22
Explanation: (100) + (101) + (110) + (111) = 4 + 5 + 6 + 7 = 22

Note:

1. The number of nodes in the tree is between 1 and 1000.
2. node.val is 0 or 1.
3. The answer will not exceed 2^31 - 1

https://leetcode.com/problems/sum-of-root-to-leaf-binary-numbers/discuss/270025/JavaC%2B%2BPython-Recursive-Solution

We recursively pass the current value of path to the children.
If root == null, no value, return 0.
If root != null,
we double the value from its parent and add the node's value,
like the process of transforming base 2 to base 10.

In the end of recursion,
if root.left == root.right == null,
return the current val.

Time Complexity:

O(N) time, O(logN) for recursion.

    int mod = 1000000007;
    public int sumRootToLeaf(TreeNode root) {
        return dfs(root, 0);
    }

    public int dfs(TreeNode root, int val) {
        if (root == null) return 0;
        val = (val * 2 + root.val) % mod;
        return (root.left == root.right ? val : dfs(root.left, val) + dfs(root.right, val)) % mod;
    }

X> https://leetcode.com/problems/sum-of-root-to-leaf-binary-numbers/discuss/270770/JAVA-SOLUTION-BEAT-100

    public int sumRootToLeaf(TreeNode root) {
        List<String> list = new ArrayList<>();
        helper(list, root, new StringBuilder().append(Integer.toString(root.val)));
        int res = 0;
        for (String ele: list) {
            res += Integer.parseInt(ele, 2);
        }
        return res;
    }
    private void helper(List<String> list, TreeNode root, StringBuilder sb) {
        if (root == null) return;
        
        if (root.left == null && root.right == null) {
            list.add(sb.toString());
            return;
        }
        
        if (root.left != null) {
            sb.append(Integer.toString(root.left.val));
            helper(list, root.left, sb);
            sb.deleteCharAt(sb.length() - 1);
        }
        if (root.right != null) {
            sb.append(Integer.toString(root.right.val));
            helper(list, root.right, sb);
            sb.deleteCharAt(sb.length() - 1);
        }
    }