6 new challenges

Ranked mode just got 6 new challenges, 4 hard and 2 medium ones.
I think the most interesting one is the implementation of an LRU (least recently used) cache structure. There are also 3 challenges related to the reverse Polish notation, which is a must-know for software engineers.

Below is a mysterious function, can you solve it?

function singleNumber(nums) {
  let ret = 0;
  for (let index = 0; index < nums.length; index++) {
    const element = nums[index];
    ret = ret ^ element;
  }
  return ret;
}

let A = singleNumber([2,3,4,3,2,5,1,5]);
// A = ?