编写函数获取两个字符串的最长公共子串，例如: s1 = GCCCTAGCCAGDE，s2 = GCGCCAGTGDE，它们的最长公共子串是 GCCAG。

getmaxsub.js

function getMaxSub(s1, s2) {
    var max = 0, retchars = [];
    for(var index = 0; index < s1.length; index++){
        var count = 0, chars = []; // 每次遍历都置空
        debug('============%d============', index);
        for(var i = index, j = 0; j < s2.length; j++){
            var s1char = s1[i];
            var s2char = s2[j];
            debug(s1char, s2char, s1char === s2char);
            if(s1char === s2char){
                count++;
                i++;
                chars.push(s1char);
            } else if(count) { // 如果不相同且有相同过则count和chars置空，i复位，j回退一位
                j--;
                count = 0;
                chars = [];
                i = index;
            }
            debug('max:%d, count:%d, i:%d, j:%d', max, count, i, j);
            if(count > max) {
                max = count;
                debug(chars);
                retchars = chars.slice();
            }
        }
    }
    return retchars.join('');
}

// =================== test ===================

var arr = [
    [
        'GCCCTAGCCAGDE', 
        'GCGCCAGTGDE',
        'GCCAG' // middle
    ], 
    [
        'GCGCCAGTGDE',
        'GCCCTAGCCAGDE', 
        'GCCAG' // middle
    ], 
    [
        'GCCCTAGCCAGDE', 
        'GCCCTAGCCA',
        'GCCCTAGCCA' // prefix
    ],
    [
        'GCCCTAGCCAGDE', 
        'GCCAGDE',
        'GCCAGDE' // suffix
    ],
    [
        'AGCCCTAGCCAGDE', 
        'BCGCCCTAGCCA',
        'GCCCTAGCCA' // mix
    ],
    [
        'GCCCTAGCCAGDE', 
        'ZZZXXX',
        '' // no common
    ],
    [
        'abcdefghijklmn',
        'abcdefghijklmn',
        'abcdefghijklmn' // same
    ],
    [
        'abcdefghijklmn',
        'n',
        'n' // single char
    ],
    [
        '',
        'n',
        '' // blank
    ],
    [
        '',
        '',
        '' // blank
    ]
];

function debug(msg){
    if(debug.on){
        console.info.apply(this, arguments);
    }
}
debug.on = 0;
if(0){
    var item = arr[3];
    var result = getMaxSub(item[0], item[1]);
    console.info(result, item[2]);
    console.info((result == item[2]) ? 'TRUE' : 'FALSE', item[0], item[1], item[2]);
    process.exit();
}

for(var i = 0; i < arr.length; i++) {
    var item = arr[i];
    var result = getMaxSub(item[0], item[1]);
    console.info((result == item[2]) ? 'TRUE' : 'FALSE', item[0], item[1], item[2]);
}