leetcode：503. 下一个更大元素 II（单调栈）

链接：https://leetcode-cn.com/problems/next-greater-element-ii/
单调栈问题，对数组操作两次即可。
java代码：

class Solution {
    public int[] nextGreaterElements(int[] nums) {
        int [] result = new int[nums.length];
        for(int i = 0;i<result.length;i++)
            result[i] = -1;
        Stack<Integer>stack = new Stack<>();
        for(int i = 0;i<nums.length;i++)
        {
            if(stack.isEmpty())
                stack.push(i);
            else
            {
                while(!stack.isEmpty()&&nums[i]>nums[stack.peek()])
                {
                    int temp = stack.pop();
                    result[temp] = nums[i];
                }
                stack.push(i);
            }
        }
        for(int i = 0;i<nums.length;i++)
        {
            if(stack.isEmpty())
                stack.push(i);
            else
            {
                while(!stack.isEmpty()&&nums[i]>nums[stack.peek()])
                {
                    int temp = stack.pop();
                    result[temp] = nums[i];
                }
                stack.push(i);
            }
        }
        return result;
    }
}