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 3The 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
.
---
Solution: Recurse down the tree, but don't recurse onto null nodes. For each recursion, multiply the previous value by 10 and add the current node's value. When you reach a leaf node, return the value. If you hit a non-leaf node, take the sum of the left and right children, but count null children nodes as zero.
/**
* 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) {
if (root == null) return 0;
return sum(root, 0);
}
private int sum(TreeNode node, int n) {
int i = (n * 10) + node.val;
if (node.left == null && node.right == null) return i;
int s = 0;
if (node.left != null) s += sum(node.left, i);
if (node.right != null) s += sum(node.right, i);
return s;
}
}
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