Leetcode 496. Next Greater Element I

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

解题思路:单调栈(维护一个从小到大的栈(栈顶到栈底))，用hash记录每一个数的下一个数。

参考代码:

public class Solution {
    public int[] nextGreaterElement(int[] findNums, int[] nums) {
        if(findNums == null || nums == null) return null;
        if(findNums.length == 0 || nums.length == 0) return new int[0];
        int[] ans = new int[findNums.length];
        Mapmap = new HashMap();
        Deque stack = new LinkedList<>();
        for(int i = nums.length-1;i >= 0; i--){
            if(stack.isEmpty()){
                stack.add(nums[i]);
                map.put(nums[i],-1);
            }
            else{
                if(nums[i] > stack.peek()){
                    while(!stack.isEmpty()){
                        if(nums[i] < stack.peek()){
                            break;
                        }
                        stack.pop();
                    }
                }
                if(stack.isEmpty()){
                    map.put(nums[i],-1);
                }
                else{
                    map.put(nums[i],stack.peek());   
                }
                stack.push(nums[i]);
            }
        }
        for(int i = 0;i < findNums.length; i++){
            ans[i] = map.get(findNums[i]);
        }
        return ans;
    }
}