Sum Root to Leaf Numbers is a popular interview problem which is asked in companies like Google & Microsoft. You can find this question in LeetCode & InterviewBit.
We have been given a binary tree where each node can contain any number between 09.
For the above tree, there are two root to leaf paths, 2>4 = 24 & 2>8 = 28. So result should be 24 + 28 = 52. In InterviewBit problem, we need to get the remainder after dividing the result by 1003 (result % 1003) to restrict number overflow. Number overflow can happen if tree has a lot of depth.
Below you can find the working Java solution for this problem. It will do a preorder traversal & do a backtracking to find all possible solutions & add them to result. Time complexity is O(n) as we need to traverse the full tree once. Space complexity is constant O(1) if we don’t consider recursion stack.
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import java.util.*; import java.lang.*; import java.io.*; import java.math.*;
//Definition for binary tree class TreeNode { int val; TreeNode left; TreeNode right; TreeNode(int x) { val = x; left=null; right=null; } }
class Solution { public int sumNumbers(TreeNode A) { String currentSum = ""; //Like Integer, BigInteger object is also immutable. Assigning new object refrence //inside method won't change method argument. So using an array of BigInteger. BigInteger[] result = new BigInteger[]{BigInteger.valueOf(0)}; calculateSumRootToLeaf(A, currentSum, result); result[0] = result[0].remainder(BigInteger.valueOf(1003)); return result[0].intValue(); } private void calculateSumRootToLeaf(TreeNode node, String currentSum, BigInteger[] result){ currentSum = currentSum + "" + node.val; if(node.left == null && node.right == null){ result[0] = result[0].add(new BigInteger(currentSum)); } else { if(node.left != null){ calculateSumRootToLeaf(node.left, currentSum, result); } if(node.right != null){ calculateSumRootToLeaf(node.right, currentSum, result); } } currentSum = currentSum.substring(0, currentSum.length()  1); } }
