LeetCode 496. Next larger element I

496. Next bigger element I

 

【 Monotonic stack 】 Go back and forth , For elements i, Keep popping than i Small elements , If the stack is not empty , Then the top of the stack is the first ratio i Big elements , And then put i Push into the stack .

class Solution {

    //  Monotone stack finds the first element larger than him on the right 
    // 1:55 2:14

    public int[] nextGreaterElement(int[] nums1, int[] nums2) {
        Deque<Integer> stack = new LinkedList();
        Map<Integer, Integer> map = new HashMap();
        for (var i = 0; i < nums2.length; i++) {
            map.put(nums2[i], i);
        }
        int[] arr = new int[nums2.length];
        for (var i = nums2.length - 1; i >= 0; i--) {
            while (!stack.isEmpty()) {
                int top = stack.peek();
                if (top > nums2[i]) {
                    arr[i] = top;
                    stack.push(nums2[i]);
                    break;
                } else {
                    stack.poll();
                }
            }
            if (stack.isEmpty()) {
                arr[i] = -1;
                stack.push(nums2[i]);
            }
        }
        int[] ans = new int[nums1.length];
        for (var i = 0; i < nums1.length; i++) {
            ans[i] = arr[map.get(nums1[i])];
        }
        return ans;
    }
}