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496. Next Greater Element I 👍

  • Time: $O(n)$
  • Space: $O(n)$
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class Solution {
 public:
  vector<int> nextGreaterElement(vector<int>& nums1, vector<int>& nums2) {
    vector<int> ans;
    unordered_map<int, int> numToNextGreater;
    stack<int> stack;  // a decreasing stack

    for (const int num : nums2) {
      while (!stack.empty() && stack.top() < num)
        numToNextGreater[stack.top()] = num, stack.pop();
      stack.push(num);
    }

    for (const int num : nums1)
      if (const auto it = numToNextGreater.find(num);
          it != numToNextGreater.cend())
        ans.push_back(it->second);
      else
        ans.push_back(-1);

    return ans;
  }
};

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class Solution {
  public int[] nextGreaterElement(int[] nums1, int[] nums2) {
    List<Integer> ans = new ArrayList<>();
    Map<Integer, Integer> numToNextGreater = new HashMap<>();
    Deque<Integer> stack = new ArrayDeque<>(); // a decreasing stack

    for (final int num : nums2) {
      while (!stack.isEmpty() && stack.peek() < num)
        numToNextGreater.put(stack.pop(), num);
      stack.push(num);
    }

    for (final int num : nums1)
      if (numToNextGreater.containsKey(num))
        ans.add(numToNextGreater.get(num));
      else
        ans.add(-1);

    return ans.stream().mapToInt(Integer::intValue).toArray();
  }
}

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class Solution:
  def nextGreaterElement(self, nums1: list[int], nums2: list[int]) -> list[int]:
    numToNextGreater = {}
    stack = []  # a decreasing stack

    for num in nums2:
      while stack and stack[-1] < num:
        numToNextGreater[stack.pop()] = num
      stack.append(num)

    return [numToNextGreater.get(num, -1) for num in nums1]