https://leetcode.com/problems/sum-of-left-leaves/ /** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */ public class Solution { private int impl(TreeNode root, boolean left) { if (root == null) { return 0; } if (left && root.left == null && root.right == null) { return root.val; } return impl(root.left, true) + impl(root.right, false); } public int sumOfLeftLeaves(TreeNode root) { return impl(root, false); } }