算法之分治法

    public AbstractStringBuilder reverse() {
        boolean hasSurrogate = false;
        int n = count - 1;
        for (int j = (n-1) >> 1; j >= 0; --j) {
            char temp = value[j];
            char temp2 = value[n - j];
            if (!hasSurrogate) {
                hasSurrogate = (temp >= Character.MIN_SURROGATE && temp <= Character.MAX_SURROGATE)
                    || (temp2 >= Character.MIN_SURROGATE && temp2 <= Character.MAX_SURROGATE);
            }
            value[j] = temp2;
            value[n - j] = temp;
        }
        if (hasSurrogate) {
            // Reverse back all valid surrogate pairs
            for (int i = 0; i < count - 1; i++) {
                char c2 = value[i];
                if (Character.isLowSurrogate(c2)) {
                    char c1 = value[i + 1];
                    if (Character.isHighSurrogate(c1)) {
                        value[i++] = c1;
                        value[i] = c2;
                    }
                }
            }
        }
        return this;
    }
/**
*    'abc defg';
*    count = 7-1;
*    n=6
*
*    j == 3;
*
*    temp = 'd'; j = 3 
*    temp2 = 'd'; n-j = 3
*    temp[3] = 'd';
*    temp[3] ='d';
*
*    j == 2;
*    temp = 'c'; j=2
*    temp = 'e'; n-j = 4
*    temp[2] = 'e';
*    temp[5] = 'c';
*
*    j == 1;
*    temp = 'b'; j=1
*    temp2 = 'f'; n-j = 5
*    temp[1] = 'f';
*    temp[6] = 'b';
*
*    j == 0;
*    temp[6] = 'a';
*    temp = 'a'; j=0
*    temp2 = 'g'; n-j = 6;
*    temp[0] = 'g';
*    temp[6] = 'a';
**/