Stack Algorithm

03 Aug 2021 in Algorithm Pageviews

Stack Algorithm!

Next Greater Element II

Given a circular integer array nums (i.e., the next element of nums[nums.length - 1] is nums[0]), return the next greater number for every element in nums.

The next greater number of a number x is the first greater number to its traversing-order next in the array, which means you could search circularly to find its next greater number. If it doesn’t exist, return -1 for this number.

Example 1:

Input: nums = [1,2,1]
Output: [2,-1,2]
Explanation: The first 1's next greater number is 2; 
The number 2 can't find next greater number. 
The second 1's next greater number needs to search circularly, which is also 2.

Example 2:

Input: nums = [1,2,3,4,3]
Output: [2,3,4,-1,4]

class Solution1 {
public:
    vector<int> nextGreaterElements(vector<int>& nums) {
        ::stack<int> stk;
        ::vector<int> vec(nums.size());
        for (int i = nums.size() - 1; i >= 0; i--) {
            stk.push(nums[i]);
        }
        for (int j = nums.size() - 1; j >= 0; j--) {
            while (!stk.empty() && (nums[j] >= stk.top())) {
                stk.pop();
            }
            vec[j] = stk.empty() ? -1 : stk.top();
            stk.push(nums[j]);
        }
        return vec;
    }
};

int main()
{
    Solution1 sol;
    vector<int> nums { 1, 2, 3, 4, 3 };
    vector<int> res;
    res = sol.nextGreaterElements(nums);
    for (const auto &value : res) {
        cout << value << " ";
    }
    return 0;
}