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Leetcode503-nextGreaterElementII

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Solution Report of LeetCode Acceptted

Description

Given a circular array (the next element of the last element is the first element of the array), print the Next Greater Number for every element. 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, output -1 for this number.

Example

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Input: [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.

Note: The length of given array won’t exceed 10000.

Solution

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class Solution {
    public int[] nextGreaterElements(int[] nums) {
        if (nums == null || nums.length == 0) return new int[0];
        int[] res = new int[nums.length];
        
        Stack<Integer> st = new Stack<>();
        
        for (int i = nums.length - 1; i >= 0; i--) st.push(i);
        
        for (int i = nums.length - 1; i >= 0; i--){
            res[i] = -1;
            while(!st.isEmpty() && nums[i] >= nums[st.peek()]) st.pop();
            if (!st.isEmpty()) res[i] = nums[st.peek()];
            st.push(i);
        }
        
        return res;
    }
}