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Formatted question description: https://leetcode.ca/all/496.html

496. Next Greater Element I (Easy)

You are given two arrays (without duplicates) nums1 and nums2 where nums1’s elements are subset of nums2. Find all the next greater numbers for nums1's elements in the corresponding places of nums2.

The Next Greater Number of a number x in nums1 is the first greater number to its right in nums2. If it does not exist, output -1 for this number.

Example 1:

Input: nums1 = [4,1,2], nums2 = [1,3,4,2].
Output: [-1,3,-1]
Explanation:
    For number 4 in the first array, you cannot find the next greater number for it in the second array, so output -1.
    For number 1 in the first array, the next greater number for it in the second array is 3.
    For number 2 in the first array, there is no next greater number for it in the second array, so output -1.

Example 2:

Input: nums1 = [2,4], nums2 = [1,2,3,4].
Output: [3,-1]
Explanation:
    For number 2 in the first array, the next greater number for it in the second array is 3.
    For number 4 in the first array, there is no next greater number for it in the second array, so output -1.

Note:

  1. All elements in nums1 and nums2 are unique.
  2. The length of both nums1 and nums2 would not exceed 1000.

Companies:
Amazon, Bloomberg, Google

Related Topics:
Stack

Similar Questions:

Solution 1. Monotonic Stack

// OJ: https://leetcode.com/problems/next-greater-element-i/
// Time: O(N)
// Space: O(N)
class Solution {
public:
    vector<int> nextGreaterElement(vector<int>& findNums, vector<int>& nums) {
        stack<int> s;
        unordered_map<int, int> m;
        for (int n : nums) {
            while (s.size() && s.top() < n) {
                m[s.top()] = n;
                s.pop();
            }
            s.push(n);
        }
        vector<int> ans;
        for (int n : findNums) ans.push_back(m.count(n) ? m[n] : -1);
        return ans;
    }
};

Java

  • class Solution {
    public int[] nextGreaterElement(int[] nums1, int[] nums2) {
        int length1 = nums1.length, length2 = nums2.length;
        List<Integer> list2 = new ArrayList<Integer>();
        for (int i = 0; i < length2; i++)
            list2.add(nums2[i]);
        int[] nextGreaterElements1 = new int[length1];
        int[] nextGreaterElements2 = new int[length2];
        Stack<Integer> stack = new Stack<Integer>();
        for (int i = 0; i < length2; i++) {
            int num = nums2[i];
            while (!stack.isEmpty() && stack.peek() < num) {
                int prevNum = stack.pop();
                int index = list2.indexOf(prevNum);
                nextGreaterElements2[index] = num;
            }
            stack.push(num);
        }
        while (!stack.isEmpty()) {
            int num = stack.pop();
            int index = list2.indexOf(num);
            nextGreaterElements2[index] = -1;
        }
        for (int i = 0; i < length1; i++) {
            int num = nums1[i];
            int index = list2.indexOf(num);
            nextGreaterElements1[i] = nextGreaterElements2[index];
        }
        return nextGreaterElements1;
    }
}

  • // OJ: https://leetcode.com/problems/next-greater-element-i/
// Time: O(A + B)
// Space: O(B) extra space
class Solution {
public:
    vector<int> nextGreaterElement(vector<int>& A, vector<int>& B) {
        unordered_map<int, int> m;
        stack<int> s;
        for (int n : B) {
            while (s.size() && n > s.top()) {
                m[s.top()] = n;
                s.pop();
            }
            s.push(n);
        }
        vector<int> ans;
        for (int n : A) ans.push_back(m.count(n) ? m[n] : -1);
        return ans;
    }
};

  • class Solution:
    def nextGreaterElement(self, nums1: List[int], nums2: List[int]) -> List[int]:
        m = {}
        stk = []
        for v in nums2:
            while stk and stk[-1] < v:
                m[stk.pop()] = v
            stk.append(v)
        return [m.get(v, -1) for v in nums1]

############

class Solution(object):
  def nextGreaterElement(self, findNums, nums):
    """
    :type findNums: List[int]
    :type nums: List[int]
    :rtype: List[int]
    """
    d = {}
    ans = [-1] * len(findNums)
    for i, num in enumerate(findNums):
      d[num] = i
    stack = []
    for num in nums:
      while stack and stack[-1] < num:
        top = stack.pop()
        if top in d:
          ans[d[top]] = num
      stack.append(num)
    return ans

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