In this Leetcode Next Greater Element I problem solution The next greater element of some element x in an array is the first greater element that is to the right of x in the same array.

You are given two distinct 0-indexed integer arrays nums1 and nums2, where nums1 is a subset of nums2.

For each 0 <= i < nums1.length, find the index j such that nums1[i] == nums2[j] and determine the next greater element of nums2[j] in nums2. If there is no next greater element, then the answer for this query is -1.

Return an array ans of length nums1.length such that ans[i] is the next greater element as described above.

Leetcode Next Greater Element I problem solution


Problem solution in Python.

class Solution:
    def nextGreaterElement(self, nums1: List[int], nums2: List[int]) -> List[int]:
        stack = []
        next_greatest = {}
        for i in range(len(nums2)): 
            while len(stack) > 0 and stack[-1][0] < nums2[i]: 
                val, index = stack.pop()
                next_greatest[val] = nums2[i]      
            stack.append((nums2[i], i))
        
        
        result = []
        for i in range(len(nums1)): 
            if nums1[i] in next_greatest: 
                result.append(next_greatest[nums1[i]])
            else: 
                result.append(-1)
        
        return result

Problem solution in Java.

class Solution:
    def nextGreaterElement(self, nums1: List[int], nums2: List[int]) -> List[int]:
        stack = []
        next_greatest = {}
        for i in range(len(nums2)): 
            while len(stack) > 0 and stack[-1][0] < nums2[i]: 
                val, index = stack.pop()
                next_greatest[val] = nums2[i]      
            stack.append((nums2[i], i))
        
        result = []
        for i in range(len(nums1)): 
            if nums1[i] in next_greatest: 
                result.append(next_greatest[nums1[i]])
            else: 
                result.append(-1)
        
        return result


Problem solution in C++.

class Solution {
public:
    vector<int> nextGreaterElement(vector<int>& nums1, vector<int>& nums2) {      
        map<int,int> dict;
        int n = nums2.size();
        int m = nums1.size();        
        for(int i=0;i<n;i++)
        {
            dict[nums2[i]] = i;
        }       
        stack<pair<int,int>> s;       
        vector<int> greater(n,-1);       
        for(int i=0;i<n;i++)
        {
            pair<int,int> p;
            while(!s.empty() && s.top().first < nums2[i])
            {
                p = s.top();
                s.pop();
                greater[p.second] = nums2[i];
            }
            p.first = nums2[i];
            p.second = i;
            s.push(p);
        }       
        vector<int> ans(m,-1);        
        for(int i=0;i<m;i++)
        {
            ans[i] = greater[dict[nums1[i]]];
        }       
        return ans;      
    }
};