【一次过】Lintcode 1206. Next Greater Element I

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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.

样例

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.

注意事项

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

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解题思路：

将nums2数组中元素即对应的下标放入map中，方便后续查找。然后依次遍历即可。

public class Solution {
    /**
     * @param nums1: an array
     * @param nums2: an array
     * @return:  find all the next greater numbers for nums1's elements in the corresponding places of nums2
     */
    public int[] nextGreaterElement(int[] nums1, int[] nums2) {
        // Write your code here
        int[] res = new int[nums1.length];
        
        Map<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++){
            res[i] = -1;

            for(int nextIndex = map.get(nums1[i])+1 ; nextIndex<nums2.length ; nextIndex++){
                if(nums2[nextIndex] > nums1[i]){
                    res[i]=nums2[nextIndex];
                    break;
                }
            }
        }
        
        return res;
    }
}