Method flipToIdx

app/src/main/java/solver/Utils.java:146–153

(int[] flip, int length, boolean zeroSum)

    // flip
    public static int flipToIdx(int[] flip, int length, boolean zeroSum) {
        int idx = 0;
        if (zeroSum) length--;
        for (int i = 0; i < length; i++) {
            idx = idx << 1 | flip[i];
        }
        return idx;
    }

    public static void idxToFlip(int[] flip, int index, int length, boolean zeroSum) {
        int sum = 0;