Leetcode 673. Number of Longest Increasing Subsequence

Given an unsorted array of integers, find the number of longest increasing subsequence.

Example 1:

Input: [1,3,5,4,7]
Output: 2
Explanation: The two longest increasing subsequence are [1, 3, 4, 7] and [1, 3, 5, 7].

Example 2:

Input: [2,2,2,2,2]
Output: 5
Explanation: The length of longest continuous increasing subsequence is 1, and there are 5 subsequences' length is 1, so output 5.

Note: Length of the given array will be not exceed 2000 and the answer is guaranteed to be fit in 32-bit signed int.

Code (Java):

class Solution {
    public int findNumberOfLIS(int[] nums) {
        if (nums == null || nums.length == 0) {
            return 0;
        }
        
        int[] lenDP = new int[nums.length];
        int[] countDP = new int[nums.length];
        int maxLen = 1;
        
        for (int i = 0; i < nums.length; i++) {
            lenDP[i] = 1;
            countDP[i] = 1;
            for (int j = 0; j < i; j++) {
                if (nums[i] > nums[j]) {
                    if (lenDP[i] < lenDP[j] + 1) {
                        lenDP[i] = lenDP[j] + 1;
                        countDP[i] = countDP[j];
                        maxLen = Math.max(maxLen, lenDP[i]);
                    } else if (lenDP[i] == lenDP[j] + 1) {
                        countDP[i] += countDP[j];
                    }
                }
            }
        }
        
        int count = 0;
        for (int i = 0; i < lenDP.length; i++) {
            if (lenDP[i] == maxLen) {
                count += countDP[i];
            }
        }
        
        return count;
    }
}