LeetCode #132 #139 #300 2018-08-23

132. Palindrome Partitioning II

https://leetcode.com/problems/palindrome-partitioning-ii/description/

相当于计算最少步数的经典模板，不过cut数是子串数 - 1。
ahhbcz's solution
代码如下：

class Solution:
    def minCut(self, s):
        """
        :type s: str
        :rtype: int
        """
        n = len(s)
        res = list(range(-1, n))
        for i in range(n):
            for (l, r) in [(i, i), (i, i+1)]:
                while l >= 0 and r < n and s[l] == s[r]:
                    res[r+1] = min(res[r+1], res[l] + 1)
                    l -= 1
                    r += 1
        return res[-1]

139. Word Break

https://leetcode.com/problems/word-break/description/

这道题依然符合模板公式。
优化一：
将wordDict转换成set，让查询时间复杂度变为O(1).
优化二：
如果切割出的字符串比wordDict中最长的字符串还长，就不用查询了。
代码如下：

class Solution:
    def wordBreak(self, s, wordDict):
        """
        :type s: str
        :type wordDict: List[str]
        :rtype: bool
        """
        res = [True]
        max_len = max(map(len, wordDict + ['']))
        wordDict = set(wordDict)
        for i in range(1, len(s) + 1):
            res += any([res[j] and s[j:i] in wordDict for j in range(max(0, i - max_len), i)]),
        return res[-1]

300. Longest Increasing Subsequence

https://leetcode.com/problems/longest-increasing-subsequence/description/

下面的解法巧妙的利用二分查找将时间复杂度降低到O(nlogn).
Python explain the O(nlogn) solution step by step
代码如下：

class Solution:
    def lengthOfLIS(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        def binarySearch(nums, target):
            left, right = 0, len(nums) - 1
            while left <= right:
                mid = (left + right) // 2
                if nums[mid] == target:
                    return mid
                if nums[mid] < target:
                    left = mid + 1
                else:
                    right = mid - 1
            return left
        
        res = []
        for num in nums:
            index = binarySearch(res, num)
            if index == len(res):
                res.append(num)
            else:
                res[index] = num
        return len(res)

下面解法为动态规划模板解法，时间复杂度为O(n²)，遇到最后一个全为1的testcase会超时。仅供参考。
代码如下：

class Solution:
    def lengthOfLIS(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        max_value = 0
        m = len(nums)
        res = [1 for x in range(m)]
        for i in range(m):
            for j in range(i):
                if nums[j] < nums[i]:
                    res[i] = max(res[i], res[j] + 1)
            max_value = max(max_value, res[i])
        return max_value