Leetcode 838. Push Dominoes

Posted 2021-08-11 SnailTyan

文章作者：Tyan
博客：noahsnail.com  |  CSDN  |  简书

1. Description

2. Solution

**解析：**两种思路，一种思路是对所有的.，判断是否替换，如果需要替换，根据可能的情况分析替换成L还是R，通过左右双指针实现。一种思路是对所有的L和R，替换其附近需要替换的.，首先，对于L左边没有R的情况，替换.为L，对于R右边不存在L的情况，替换.为R；对于R右边存在L的情况，R和L正中间的.保持不变，左半部分变为R，右边部分变为L，循环从L的下一位重新开始。

• Version 1

class Solution:
    def pushDominoes(self, dominoes: str) -> str:
        n = len(dominoes)
        state = list(dominoes)
        for i in range(n):
            if state[i] == '.':
                left = i - 1
                right = i + 1
                while left > -1 and dominoes[left] == '.':
                    left -= 1
                while right < n and dominoes[right] == '.':
                    right += 1
                if left != -1 and right != n:
                    if dominoes[left] == 'R' and dominoes[right] == 'L':
                        if i - left > right - i:
                            state[i] = 'L'
                        elif i - left < right - i:
                            state[i] = 'R'
                    elif dominoes[left] == 'R' and dominoes[right] == 'R':
                        state[i] = 'R'
                    elif dominoes[left] == 'L' and dominoes[right] == 'L':
                        state[i] = 'L'
                elif right != n and dominoes[right] == 'L':
                    state[i] = 'L'
                elif left != -1 and dominoes[left] == 'R':
                    state[i] = 'R'

        return ''.join(state)

• Version 2

class Solution:
    def pushDominoes(self, dominoes: str) -> str:
        n = len(dominoes)
        state = list(dominoes)
        i = 0
        while i < n:
            if dominoes[i] == 'L':
                left = i - 1
                while left > -1 and dominoes[left] == '.':
                    state[left] = 'L'
                    left -= 1
            elif dominoes[i] == 'R':
                right = i + 1
                while right < n and dominoes[right] == '.':
                    state[right] = 'R'
                    right += 1
                if right != n and dominoes[right] == 'L':
                    for k in range(right - (right - i - 1) // 2, right):
                        state[k] = 'L'
                    if (right - i - 1) % 2 == 1:
                       state[i + (right - i - 1) // 2 + 1] = '.'
                    i = right
            i += 1

        return ''.join(state)

Reference

1. https://leetcode.com/problems/push-dominoes/