LC 640. Solve the Equation

Solve a given equation and return the value of x in the form of string "x=#value". The equation contains only '+', '-' operation, the variable x and its coefficient.

If there is no solution for the equation, return "No solution".

If there are infinite solutions for the equation, return "Infinite solutions".

If there is exactly one solution for the equation, we ensure that the value of x is an integer.

Example 1:

Input: "x+5-3+x=6+x-2"
Output: "x=2"

Example 2:

Input: "x=x"
Output: "Infinite solutions"

Example 3:

Input: "2x=x"
Output: "x=0"

Example 4:

Input: "2x+3x-6x=x+2"
Output: "x=-1"

Example 5:

Input: "x=x+2"
Output: "No solution"

Runtime: 4 ms, faster than 99.39% of Java online submissions for Solve the Equation.

class Solution {
  public static int[] mergenumber(String s){
    int cnt = 0, tmpcnt = 0, xval = 0, constval = 0, prev = 1;
    if(s.charAt(tmpcnt) == '+' || s.charAt(tmpcnt) == '-'){
      cnt = ++tmpcnt;
      prev = s.charAt(cnt-1) == '+' ? 1 : -1;
    }
    while(cnt < s.length()){
      while(tmpcnt < s.length() && s.charAt(tmpcnt) != '+' && s.charAt(tmpcnt) != '-'){
        tmpcnt++;
      }
      if(s.charAt(tmpcnt-1) == 'x'){
        if(tmpcnt -1 > cnt) xval += prev * Integer.parseInt(s.substring(cnt, tmpcnt-1));
        else xval += prev * 1;
      }else constval += prev * Integer.parseInt(s.substring(cnt, tmpcnt));
      if(tmpcnt == s.length()) break;
      cnt = ++tmpcnt;

      prev = s.charAt(tmpcnt-1) == '+' ? 1 : -1;
    }
    int[] ret = {xval, constval};
    return ret;
  }
  public static String solveEquation(String equation) {
    int idx = 0;

    for(; idx<equation.length(); idx++){
      if(equation.charAt(idx) == '=') break;
    }
    int[] left = mergenumber(equation.substring(0,idx));
//    System.out.println(left[0]);
//    System.out.println(left[1]);
    int[] right = mergenumber(equation.substring(idx+1));
//    System.out.println(right[0]);
//    System.out.println(right[1]);
    if(left[0] == right[0]){
      if (left[1] == right[1]) return "Infinite solutions";
      else return "No solution";
    } else {
      int ret = -1 * (right[1] - left[1]) / (right[0] - left[0]);
      String rets = "x=" + String.valueOf(ret);
      return rets;
    }
  }
}