## [Leetcode]1022. Sum of Root To Leaf Binary Numbers

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.
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## [Leetcode]1286. Iterator for Combination

Design an Iterator class, which has:
A constructor that takes a string characters of sorted distinct lowercase English letters and a number combinationLength as arguments.
A function next() that returns the next combination of length combinationLength in lexicographical order.
A function hasNext() that returns True if and only if there exists a next combination.
Constraints:
1 <= combinationLength <= characters.length <= 15
There will be at most 10^4 function calls per test.
It's guaranteed that all calls of the function next are valid.
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## [Leetcode problems]137. Single Number II

Given a non-empty array of integers, every element appears three times except for one, which appears exactly once. Find that single one.
Note:
Your algorithm should have a linear runtime complexity. Could you implement it without using extra memory?
Example 1:
Input: [2,2,3,2] Output: 3
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