[640. Solve the Equation] #leetcode

640_0901.java

class Solution {
	// O(n) O(1)
    public String solveEquation(String equation) {
        int x = 0, c = 0, num = -1, sign = 1; 
        boolean flag = false; // if at right of equal sign
        
        for (int i = 0; i < equation.length(); i++) {
            Character s = equation.charAt(i);
            if (Character.isDigit(s)) {
                if (num == -1) {
                    num = Character.getNumericValue(s);
                } else {
                    num = 10*num + Character.getNumericValue(s);
                }
            }
            if (s.equals('x')) {
                if (flag) {
                    x -= sign*(num == -1? 1 : num);
                } else {
                    x += sign*(num == -1? 1 : num);
                }
                num = -1;
            }
            if (s.equals('+')) {
                if (num != -1) {
                    if (flag) {
                        c += sign*num;
                    } else {
                        c -= sign*num;
                    }
                    num = -1;
                }
                sign = 1;
            }
            if (s.equals('-')) {
                if (num != -1) {
                    if (flag) {
                        c += sign*num;
                    } else {
                        c -= sign*num;
                    }
                    num = -1;
                }
                sign = -1;
            }
            if (s.equals('=')) {
                if (num != -1) {
                    if (flag) {
                        c += sign*num;
                    } else {
                        c -= sign*num;
                    }
                    num = -1;
                }
                flag = true;
                sign = 1;
            }
        }
        
        if (num != -1) {
            if (flag) {
                c += sign*num;
            } else {
                c -= sign*num;
            }
            num = -1;
        }
        
        System.out.printf("%d %d %d", x, c, num);
        if (x == 0 && c == 0) return "Infinite solutions";
        if (x == 0) return "No solution";
        return String.format("x=%1$d", c/x);
    }
}