本部落格記錄的是 LeetCode 71 到 90 題的題解

# 72. Edit Distance
a += ' '
a = ' ' + a

71. Simplify Path

雙指標演算法

class Solution:
    def simplifyPath(self, path: str) -> str:
        i, j = 0, 0
        n = len(path)
        dir_names = []
        while i < n:
            if path[i] == '/':
                while i < n and path[i] == '/':
                    i += 1
            else:
                j = i + 1
                while j < n and path[j] != '/':
                    j += 1
                dir_names.append(path[i:j])
                i = j

        new_dir = []
        for name in dir_names:
            if name == '.':
                continue
            elif name == '..':
                if new_dir:
                    new_dir.pop()
            else:
                new_dir.append(name)
        
        return '/' + '/'.join(new_dir)
        
 

72. Edit Distance

入門動態規劃，最短編輯距離

# Dp
class Solution:
    def minDistance(self, a: str, b: str) -> int:
        # define and initialize
        n, m = len(a), len(b)
        a = ' ' + a
        b = ' ' + b
        f = [[0] * (m + 1) for i in range(n + 1)]
        for j in range(0, m + 1):
            f[0][j] = j
        for i in range(0, n + 1):
            f[i][0] = i
        
        # dynamic programming
        for i in range(1, n + 1):
            for j in range(1, m + 1):
                if a[i] == b[j]:
                    f[i][j] = f[i - 1][j - 1]
                else:
                    f[i][j] = min(f[i - 1][j - 1], min(f[i - 1][j], f[i][j - 1])) + 1
        return f[n][m]

 

73. Set Matrix Zeroes

一定遇到原地操作，空間複雜度O(0)的時候，主要是應用好原本的陣列，僅僅開闢常數級別的變數

class Solution:
    def setZeroes(self, matrix: List[List[int]]) -> None:
        """
        Do not return anything, modify matrix in-place instead.
        """
        flag_0_row, flag_0_column = False, False
        n, m = len(matrix), len(matrix[0])
        for i in range(n):
            for j in range(m):
                if matrix[i][j] == 0:
                    matrix[i][0] = 0
                    matrix[0][j] = 0
                    if i == 0:
                        flag_0_row = True
                    if j == 0:
                        flag_0_column = True
        for i in range(1, n):
            if matrix[i][0] == 0:
                for j in range(m):
                    matrix[i][j] = 0
        for j in range(1, m):
            if matrix[0][j] == 0:
                for i in range(n):
                    matrix[i][j] = 0
        if flag_0_row:
            for j in range(m):
                matrix[0][j] = 0
        if flag_0_column:
            for i in range(n):
                matrix[i][0] = 0
 

74. Search a 2D Matrix

簡單的二分問題，先 row 二分，然後 col 二分即可，查詢是否存在 target 數值

class Solution:
    def searchMatrix(self, matrix: List[List[int]], target: int) -> bool:
        n, m = len(matrix), len(matrix[0])
        l, r = 0, n - 1
        if matrix[l][0] > target:
            return False
        else:
            # 尋找 matrix[XXX][0] <= target 的最大值
            while l < r:
                mid = (l + r + 1) // 2
                if matrix[mid][0] <= target:
                    l = mid
                else:
                    r = mid - 1
            
            # 尋找 matrix[x][XXX] <= target 的最大值
            x = l
            l, r = 0, m - 1
            while l < r:
                mid = (l + r + 1) // 2
                if matrix[x][mid] <= target:
                    l = mid
                else:
                    r = mid - 1
            return matrix[x][l] == target

75. Sort Colors

# 快排

# 或者是直接進行數數
class Solution:
    def sortColors(self, nums: List[int]) -> None:
        """
        Do not return anything, modify nums in-place instead.
        """
        cnt, n = [0] * 3, len(nums)
        for i in range(n):
            cnt[nums[i]] += 1
        sum = 0
        for k in range(3):
            for i in range(sum, sum + cnt[k]):
                nums[i] = k
            sum += cnt[k]
            

如果是隻想掃描一遍的話，可以按照雙指標演算法來寫
有 0 就從前往後放，有 2 就從後往前放，剩下的就是 1的地方，不過這樣放置的話，可能會覆蓋一個數組單元，因此需要那一個臨時變數儲存一下子，這裡我使用的 t = nums[n - 1] 來防止 nums[[n - 1]未遍歷前就被覆蓋了

class Solution:
    def sortColors(self, nums: List[int]) -> None:
        """
        Do not return anything, modify nums in-place instead.
        """
        n = len(nums)
        i, j = 0, n - 2
        t = nums[n - 1]
        cnt = 0
        pos_0, pos_2 = 0, n - 1
        flag = True
        while i <= j:
            if flag:
                if nums[i] == 0:
                    nums[pos_0] = 0
                    pos_0 += 1
                elif nums[i] == 2:
                    nums[pos_2] = 2
                    pos_2 -= 1
                    flag = False
                i += 1
            else:
                if nums[j] == 0:
                    nums[pos_0] = 0
                    pos_0 += 1
                    flag = True
                elif nums[j] == 2:
                    nums[pos_2] = 2
                    pos_2 -= 1
                j -= 1
        
        if t == 0:
            nums[pos_0] = 0
            pos_0 += 1
        elif t == 2:
            nums[pos_2] = 2
            pos_2 -= 1
        for i in range(pos_0, pos_2 + 1):
            nums[i] = 1