1022.sum-of-root-to-leaf-binary-numbers.1.java

/*
 * @lc app=leetcode id=1022 lang=java
 *
 * [1022] Sum of Root To Leaf Binary Numbers
 *
 * https://leetcode.com/problems/sum-of-root-to-leaf-binary-numbers/description/
 *
 * algorithms
 * Easy (50.29%)
 * Likes:    125
 * Dislikes: 55
 * Total Accepted:    15K
 * Total Submissions: 25.8K
 * Testcase Example:  '[1,0,1,0,1,0,1]'
 *
 * Given a binary tree, each node has value 0 or 1.  Each root-to-leaf path
 * represents a binary number starting with the most significant bit.  For
 * example, if the path is 0 -> 1 -> 1 -> 0 -> 1, then this could represent
 * 01101 in binary, which is 13.
 *
 * For all leaves in the tree, consider the numbers represented by the path
 * from the root to that leaf.
 *
 * Return the sum of these numbers.
 *
 *
 *
 * Example 1:
 *
 *
 *
 *
 * Input: [1,0,1,0,1,0,1]
 * Output: 22
 * Explanation: (100) + (101) + (110) + (111) = 4 + 5 + 6 + 7 = 22
 *
 *
 *
 *
 * Note:
 *
 *
 * The number of nodes in the tree is between 1 and 1000.
 * node.val is 0 or 1.
 * The answer will not exceed 2^31 - 1.
 *
 *
 */
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
  public int sumRootToLeaf(TreeNode root) {
    return dfs(root, 0);
  }

  public int dfs(TreeNode root, int prevSum) {
    if (root == null) return 0;

    prevSum = 2 * prevSum + root.val;
    if (root.left == null && root.right == null) {
      return prevSum;
    }

    return dfs(root.left, prevSum) + dfs(root.right, prevSum);
  }
}