【leetcode刷题】T209-求解方程

示例 1：
输入: "x+5-3+x=6+x-2"
输出: "x=2"

示例 2:
输入: "x=x"
输出: "Infinite solutions"

示例 3:
输入: "2x=x"
输出: "x=0"

示例 4:
输入: "2x+3x-6x=x+2"
输出: "x=-1"

示例 5:
输入: "x=x+2"
输出: "No solution"

class Solution(object):
    def solveEquation(self, equation):
        """
        :type equation: str
        :rtype: str
        """
        a = 0
        b = 0
        start = 0
        flag = 1
        for i in range(len(equation)):
            if equation[i] == 'x':
                # 'x'要分为单独的'x'和带系数的'ax'分别处理
                if start == i or (i - start == 1 and equation[start] == '+'):
                    a += flag
                elif i - start == 1 and equation[start] == '-':
                    a -= flag
                else:
                    a += flag * int(equation[start:i])
                start = i + 1
            elif equation[i] == '=':
                # 等号前的数字也要单独处理
                if i > start:
                    b += flag * int(equation[start:i])
                flag = -1
                start = i + 1
            elif equation[i] == '+' or equation[i] == '-':
                # print(equation[start:i])
                # 'x+2'在x后面也有个+，''不能被转换为int
                if i == start:
                    continue
                b += flag * int(equation[start:i])
                start = i
        # 结束要处理
        if start < len(equation):
            b += flag * int(equation[start:])

        if a == 0 and b == 0:
            return "Infinite solutions"
        if a == 0 and b != 0:
            return "No solution"
        return "x=" + str(-b // a)