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.

 1 /**
 2  * Definition for a binary tree node.
 3  * public class TreeNode {
 4  *     int val;
 5  *     TreeNode left;
 6  *     TreeNode right;
 7  *     TreeNode(int x) { val = x; }
 8  * }
 9  */
10 public class Solution {
11     public int sumOfLeftLeaves(TreeNode root) {
12         return helper(root, false);
13     }
14     
15     public int helper(TreeNode cur, boolean isLeft) {
16         if (cur == null) return 0;
17         if (isLeft && cur.left==null && cur.right==null) return cur.val;
18         return helper(cur.left, true) + helper(cur.right, false);
19     }
20 }