Sum Root to Leaf Numbers

Given a binary tree containing digits from 0-9 only, each root-to-leaf path could represent a number.

An example is the root-to-leaf path 1->2->3 which represents the number 123.

Find the total sum of all root-to-leaf numbers.

For example,

    1
   / \
  2   3

 

The root-to-leaf path 1->2 represents the number 12.
The root-to-leaf path 1->3 represents the number 13.

Return the sum = 12 + 13 = 25.

 

如果bfs不能处理 4,9,0，null，1 这种，答案会是891，因为40错误乘10了，正确答案是531 =491+40

用dfs（不知道是不是dfs）

    public int sumNumbers(TreeNode root) {
        if(root==null) return 0;
        return dfs(root, 0); // 0 为传递到root这个点的时候的积累值
    }
    private int dfs(TreeNode n, int preSum){
        if(n==null) return 0;
        int sum = n.val+10*preSum;
        if(n.left==null && n.right==null) return sum;
        if(n.left!=null && n.right!=null) return dfs(n.left,sum)+dfs(n.right,sum);
        if(n.left!=null) return dfs(n.left,sum);
        return dfs(n.right,sum);
    }
}