LeetCode/cs/sum-of-left-leaves.cs

#if _MF_TEST
// Definition for a binary tree node.
public class TreeNode {
    public int val;
    public TreeNode left;
    public TreeNode right;
    public TreeNode(int val = 0, TreeNode left = null, TreeNode right = null) {
        this.val = val;
        this.left = left;
        this.right = right;
    }
}
#endif

public class Solution {
    private static bool IsLeaf(TreeNode node)
        => node != null && node.left == null && node.right == null;

    private int Sum(TreeNode node, bool isLeft) {
        if (node == null) {
            return 0;
        }

        if (isLeft && IsLeaf(node)) {
            return node.val;
        }

        return Sum(node.left, true) + Sum(node.right, false);

    }

    public int SumOfLeftLeaves(TreeNode root)
        => Sum(root, false);
}