Leetcode: 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.

主要是看你如何设计这个Recursion 这是一道树的题目，一般使用递归来做，主要就是考虑递归条件和结束条件。想到这样设计Recursion其实还挺难的

public class Solution {
    public int sumNumbers(TreeNode root) {
        if (root == null) return 0;
        return helper(root, 0);
    }
    
    public int helper(TreeNode root, int sum) {
        if (root.left==null && root.right==null) return 10*sum+root.val;
        else if (root.left == null) return helper(root.right, 10*sum+root.val);
        else if (root.right == null) return helper(root.left, 10*sum+root.val);
        else {
            return helper(root.left, 10*sum+root.val) + helper(root.right, 10*sum+root.val);
        }
    }
}

推荐做法：

public int sumNumbers(TreeNode root) {
    return helper(root,0);
}
private int helper(TreeNode root, int sum)
{
    if(root == null)
        return 0;
    if(root.left==null && root.right==null)
        return sum*10+root.val;
    return helper(root.left,sum*10+root.val)+helper(root.right,sum*10+root.val);
}