129 Sum Root to Leaf Numbers (An example is the root-to-leaf path 1->2->3 which represents the number 123.)

129 Sum Root to Leaf Numbers


https://leetcode.com/problems/sum-root-to-leaf-numbers/discuss/41363/Short-Java-solution.-Recursion.



不是自己写的， tree 的 recursion 做的 不好
class Solution {
    public int sumNumbers(TreeNode root) {
      return sum(root, 0);
    }
    private int sum(TreeNode n, int prev){
      if(n == null){
        return 0;
      }
      if(n.right == null && n.left == null){
        return prev * 10 + n.val;
      }
      int left = sum(n.left, prev * 10 + n.val);
      int right = sum(n.right, prev * 10 + n.val);
      int sum = left + right;
      return sum;
        
    }
}

 

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

 

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.

Note: A leaf is a node with no children.

Example:

Input: [1,2,3]
    1
   / \
  2   3
Output: 25
Explanation:
The root-to-leaf path 1->2 represents the number 12.
The root-to-leaf path 1->3 represents the number 13.
Therefore, sum = 12 + 13 = 25.

Example 2:

Input: [4,9,0,5,1]
    4
   / \
  9   0
 / \
5   1
Output: 1026
Explanation:
The root-to-leaf path 4->9->5 represents the number 495.
The root-to-leaf path 4->9->1 represents the number 491.
The root-to-leaf path 4->0 represents the number 40.
Therefore, sum = 495 + 491 + 40 = 1026.