LeetCode - Sum of Left Leaves

Find the sum of all left leaves in a given binary tree.

Example:

    3
   / \
  9  20
    /  \
   15   7

There are two left leaves in the binary tree, with values 9 and 15 respectively. Return 24.

/**
 * 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;
        }
        return sumOfLeftLeaves(root, false);
        
    }
    
    private int sumOfLeftLeaves(TreeNode node, boolean isLeft){
        if(node == null){
            return 0;
        }
        int sum = 0;
        if(node.left != null || node.right != null){
            sum = sum + sumOfLeftLeaves(node.left, true);
            sum = sum + sumOfLeftLeaves(node.right, false);
        }
        else if(isLeft) {
            sum = sum+node.val;
        }
        return sum;
    }
    
}