Snakeless path

#include <bits/stdc++.h>
#include <vector>
#define ll long long

template <typename T>
std::ostream &operator<<(std::ostream &os, const std::vector<T> &v) {
  os << "[";
  for (int i = 0; i < v.size(); ++i) {
    os << v[i];
    if (i != v.size() - 1)
      os << ", ";
  }
  os << "]";
  return os;
}

ll twoSum(std::vector<std::pair<ll, ll>> &nums, ll target, ll illegal, ll &le,
          ll &ri) {
  //   ll le = 0, ri = nums.size();
  while (nums[le].first + nums[ri].first != target && le < ri) {
    if (nums[le].second == illegal)
      le += 1;
    if (nums[ri].second == illegal)
      ri -= 1;

    if (nums[le].first + nums[ri].first > target) {
      ri--;
    } else {
      le++;
    }
  };
  if (nums[le].second == illegal)
    return 0;
  if (nums[ri].second == illegal)
    return 0;
  if (nums[le].second == nums[ri].second)
    return 0;

  if (nums[le].first + nums[ri].first == target && nums[le].second != illegal &&
      nums[ri].second != illegal) {
    // std::cout << le << " " << ri << std::endl;
    return 1;
  }
  return 0;
}

int main() {
  ll numCount, target;
  std::cin >> numCount;
  std::cin >> target;

  std::vector<std::pair<ll, ll>> nums;
  for (ll ii = 0; ii < numCount; ii++) {
    // std::cout << start << " " << end << " " << speed << std::endl;
    ll num;
    std::cin >> num;
    nums.push_back({num, ii});
  }
  std::sort(nums.begin(), nums.end());
  for (std::pair<ll, ll> &nu : nums) {
    ll le = 0, ri = nums.size() - 1;
    ll ret = twoSum(nums, target - nu.first, nu.second, le, ri);
    if (ret) {
      //   std::cout << le << " " << ri << std::endl;
      std::cout << nums[le].second + 1 << " " << nums[ri].second + 1 << " "
                << nu.second + 1 << std::endl;
      return 0;
    }
  }
  std::cout << "IMPOSSIBLE" << std::endl;
}

Test 1

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Test 2

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Test 3

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Test 4

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Test 70

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Test 84

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Test 86

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Test 88

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Test 89

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