404. Sum of Left Leaves

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

    }
}