Sum Root to Leaf Numbers

lc 129

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.

碰到tree recursive要不要叶子节点，所以都是先进来判断root 是不是空，然后判断要不要把这个要求加进去，然后再recursive的处理。

题目解答

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public int sumNumbers(TreeNode root) {
        int[] result = new int[1];
        StringBuilder sb = new StringBuilder();
        if(root == null) return 0;

        helper(result, sb, root);
        return result[0];

    }

    public void helper(int[] result, StringBuilder sb, TreeNode root){
       // if(root == null) return;
        sb.append(root.val);
        if(root.left == null && root.right == null){
            int num = Integer.valueOf(sb.toString());
            result[0] += num;
            //sb.deleteCharAt(sb.length() - 1);
            return;
        }
        if(root.left != null){
            helper(result, sb, root.left);
               sb.deleteCharAt(sb.length() - 1);
        }
        if(root.right != null){
            helper(result, sb, root.right);
               sb.deleteCharAt(sb.length() - 1);

        }

    }
}