Leetcode-Number of Longest Increasing Subsequence

Description

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.

Explain

最近都会做动态规划的题，毕竟动态规划的核心思想是
状态 和 状态方程。当前的状态依赖上一个状态，我们要解这类题就得找出状态转移方程，放在高中就是递推公式。而这道题，和LIS挺类似的。这道题不是求最长递增子串的长度了，是求它的数量了。而我们求出当前的最长长度时，那它的数量就是 maxLen - 1 的数量和，状态转移方程就出来了：dpp[i] = max(dpp[i], dpp[j]+...+dpp[k])(j...k 为dp值为dp[i] - 1的下标)。那么我们就可以用一个dpp数组来记录对应的数量，最后算出最长长度时，遍历一遍数组将dp[i]为最大值的对应的dpp[i]相加就得出结果了。还是看代码吧

Code

class Solution {
public:
    int findNumberOfLIS(vector& nums) {
        if (nums.empty()) return 0;
        int res = 0;
        int maxx = INT_MIN;
        vector dp(nums.size(), 1);
        vector dpp(nums.size(), 1);
        for (int i = 0; i < nums.size(); i++) {
            int count = 0;
            for (int j = 0; j < i; j++) {
                if (nums[i] > nums[j]) {
                    dp[i] = max(dp[j] + 1, dp[i]);
                }
            }
            
            for (int j = 0; j < i; j++) {
                if (dp[i] == 1) count = 1;
                else if (dp[j] == dp[i] - 1 && nums[j] < nums[i]) {
                    count += dpp[j];
                }
            }
            
            dpp[i] = max(dpp[i], count);
            
            maxx = max(maxx, dp[i]);
        }
        for (int i = 0; i < nums.size(); i++) {
            if (maxx == dp[i]) res += dpp[i];
        }
        return res;
    }
};