404-Sum-of-Left-Leaves.java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public int sumOfLeftLeaves(TreeNode root) {
        
        if (root == null)
            return 0;

        int counter = 0;

        if (root.left != null && root.left.left == null && root.left.right == null) 
            counter += root.left.val;

        counter += sumOfLeftLeaves(root.left);
        counter += sumOfLeftLeaves(root.right);

        return counter;

        
    }
}