[Leetcode]404. Sum of Left Leaves

/**
* 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;
}
int sum = 0;

if (root.left != null && isLeaf(root.left) == true) {
sum += root.left.val;
}
if (root.left != null && isLeaf(root.left) == false) {
sum = sum + sumOfLeftLeaves(root.left);
}
if (root.right != null && isLeaf(root.right) == false) {
sum = sum + sumOfLeftLeaves(root.right);
}
return sum;
}

public boolean isLeaf(TreeNode root) {
if (root != null && root.right == null && root.left == null) {
return true;
} else {
return false;
}
}
}