二叉树的深度搜索专题一＞求根节点到叶节点数字之和

 public int sumNumbers(TreeNode root) {
        return dfs(root,0);
    }

    private int dfs(TreeNode root, int preSum){
        //1.传入的前驱
        preSum = preSum * 10 + root.val;

        //2.叶子节点
        if(root.left == null && root.right == null)
            return preSum;

        //3.非叶子节点
        int ret = 0;
        if(root.left != null) ret += dfs(root.left,preSum);
        if(root.right != null) ret += dfs(root.right,preSum);
        return ret;
    }