[LeetCode] Next Greater Element I

Posted 2020-09-27 immjc

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:

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

首先利用map记录nums中元素及其对应的索引，方便查找。接着在遍历findNums中的元素，根据其在map中相应元素的位置查找下一个较大的数。最后如果找到符合要求的数，跳出循环，将这个数字替换res中对应位置的-1。

class Solution {
public:
    vector<int> nextGreaterElement(vector<int>& findNums, vector<int>& nums) {
        vector<int> res(findNums.size(), -1);
        unordered_map<int, int> m;
        for (int i = 0; i != nums.size(); i++)
            m[nums[i]] = i;
        for (int i = 0; i != findNums.size(); i++) {
            int start = m[findNums[i]];
            for (int j = start + 1; j != nums.size(); j++) 
                if (nums[j] > findNums[i]) {
                    res[i] = nums[j];
                    break;
                }
        }
        return res;
    }
};
// 9 ms