9.19力扣每日一題——左子樹和

public class TreeNode {
      int val;
      TreeNode left;
      TreeNode right;
      TreeNode(int x) { val = x; }
  }
 
class Solution {
    int sum = 0;
    public int sumOfLeftLeaves(TreeNode root) {
        dfs(root);
        return sum;
    }

    public void dfs(TreeNode root){  //遞歸
        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);
    }
}