# 0496. Next Greater Element I
###### tags: `Leetcode` `Easy` `Stack` `FaceBook`
Link: https://leetcode.com/problems/next-greater-element-i/
## 思路 O(N) O(N)
先用map存，nums2里面的每个元素和它next greater element的对应
然后再用回圈遍历nums1，找到答案
## Code

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