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 {
    private int sum;
    public int sumOfLeftLeaves(TreeNode root) {
        if (root == null)
            return 0;
        if (root.left == null && root.right == null)
            return 0;
        solve(root);
        return sum;
    }
    private void solve(TreeNode root) {
        if (root == null)
            return ;
        if (root.left != null) {
            if (root.left.left == null && root.left.right == null)
                sum += root.left.val;
        }
        solve(root.left);
        solve(root.right);
    }
}