Solution.java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int sumOfLeftLeaves(TreeNode root) {
        if (root == null) {
            return 0;
        } else if (root.left == null && root.right == null) {
            return 0;
        }
        return sumOfLeftLeaves(root.left, true) + sumOfLeftLeaves(root.right, false);
    }
    
    public int sumOfLeftLeaves(TreeNode node, boolean isLeft) {
        if (node == null) {
            return 0;
        } else if (node.left == null && node.right == null && isLeft) {
            return node.val;
        }
        return sumOfLeftLeaves(node.left, true) + sumOfLeftLeaves(node.right, false);
    }
}