https://leetcode.com/problems/solve-the-equation/

solve-the-equation.java

class Solution {
    public String solveEquation(String equation) {
        String[] tokens = equation.split("=");
        if (tokens.length != 2) {
            return "No solution";
        }
        int[] left = parse(tokens[0]);
        int[] right = parse(tokens[1]);
        
        // move x to left, num to right
        int x = left[1] - right[1];
        int num = right[0] - left[0];
        
        if (x == 0) {
            if (num == 0) {
                return "Infinite solutions";
            } else {
                return "No solution";
            }
        }
        return "x=" + num/x; 
    }
    
    
    private int[] parse(String s) {
        // int[] -> {num, x}    
        int num = 0;
        boolean seeNum = false;
        int x = 0;
        
        int val = 0;
        int sign = 1;
        int i = 0;
        
        while(i < s.length()) {
            if (Character.isDigit(s.charAt(i))) {
                val = val * 10 + s.charAt(i) - '0';
                seeNum = true;
            } else if (s.charAt(i) == 'x') {
                x += (val > 0 || seeNum ? sign * val : sign);
                
                val = 0;
            } else if (s.charAt(i) == '+') {
                num += (sign * val);
                val = 0;
                sign = 1;
                seeNum = false;
            } else if (s.charAt(i) == '-') {
                num += (sign * val);
                val = 0;
                sign = -1;
                seeNum = false;
            }
            i++;
        }
        if (val > 0) {
            num += (sign * val);
        }
        return new int[]{num, x};
    }
}