Sum of Left Leaves

3
/ \
9  20
/  \
15   7

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

public class SumofLeftLeaves {
public int sumOfLeftLeaves(TreeNode root) {
int sum = 0;
if (root == null)
return 0;
if (isLeave(root.left))
sum += root.left.val;
sum += sumOfLeftLeaves(root.left);
sum += sumOfLeftLeaves(root.right);

return sum;
}

public  boolean isLeave(TreeNode node) {
if (node == null)
return false;
if (node.left == null && node.right == null)
return true;
return false;
}
}