404. 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; }
 * }
 */
public class Solution  
    public int sumOfLeftLeaves(TreeNode root) {  
        if (root == null)  
            return 0;  
        if (root.left != null && root.left.right == null  
                && root.left.left == null) {  
            return root.left.val + sumOfLeftLeaves(root.right);  
        }  
        return