496.Next Greater Element I

1-HashMap + Linear Search

class Solution {
    public int[] nextGreaterElement(int[] nums1, int[] nums2) {
        int n = nums2.length;
        int[] ans = new int[nums1.length];
        HashMap<Integer, Integer> map = new HashMap<>();
        for(int i = 0; i < nums2.length; i++){
            map.put(nums2[i], i);
        }
        for(int i = 0 ; i < nums1.length; i++){
            int pos = map.get(nums1[i]);
            boolean isFind = false;
            for(int j = pos + 1; j < n; j++){
                if(nums2[j] > nums1[i]){
                    isFind = true;
                    ans[i] = nums2[j];
                    break;
                }
            }
            if(!isFind)
                ans[i] = -1;
        }
        return ans;
    }
}

2-Stack

https://leetcode.com/problems/next-greater-element-i/solution/

class Solution {
    public int[] nextGreaterElement(int[] nums1, int[] nums2) {
        Deque<Integer> stack = new ArrayDeque<>();
        Map<Integer, Integer> map = new HashMap<>();
        int[] ans = new int[nums1.length];
        for(int i = 0; i < nums2.length; i++){
            while(!stack.isEmpty() && nums2[i] > stack.peek())
                map.put(stack.pop(), nums2[i]);
            stack.push(nums2[i]);
        }
        while(!stack.isEmpty()){
            map.put(stack.pop(), -1);
        }
        for(int i = 0; i < nums1.length; i++){
            ans[i] = map.get(nums1[i]);
        }
        return ans;
    }
}