565.cpp

// beat96%+O1Space.cpp
class Solution {
public:
  int arrayNesting(vector<int> &nums) {
    int n = nums.size(), maxlen = 0;
    for (int i = 0; i < n; ++i) {
      int value = nums[i], pos = i, len = 0;
      while (value != -1) {
        nums[pos] = -1;
        pos = value;
        value = nums[value];
        ++len;
      }
      maxlen = max(maxlen, len);
      if (maxlen > n / 2)
        return maxlen;
    }
    return maxlen;
  }
};
// hashset.cpp
class Solution2 {
public:
  int arrayNesting(vector<int> &nums) {
    unordered_set<int> hashset;
    int len = 0, n = nums.size(), maxlen = 0;
    for (int i = 0; i < n; ++i) {
      int value = nums[i];
      while (hashset.find(value) == hashset.end()) {
        hashset.insert(value);
        value = nums[value];
        ++len;
      }
      maxlen = max(maxlen, len);
      len = 0;
    }
    return maxlen;
  }
};