统计字符串中字符出现次数（延伸）

returnStrNum: function(str, who) {
    var strNum = {};
    for (var i = 0, l = str.length; i < l; i++) {
        strNum[str[i]] = (strNum[str[i]] + 1) || 1;
    }
    return strNum[who];
},
console.log(that.returnStrNum('sgdgfdgrrgdv', 'g'));//4

returnStrNum: function(str) {
    var strNum = {};
    var arr = [];
    var keys = 1;
    for (var i = 0, l = str.length; i < l; i++) {
        strNum[str[i]] = (strNum[str[i]] + 1) || 1;
    }
    for (const key in strNum) {
        if (strNum.hasOwnProperty(key)) {
            const element = strNum[key];
            if (element > keys) {
                arr = key;
                keys = element;
            }
        }
    }
    return arr + ':' + keys;

},
console.log(that.returnStrNum('sgdgfdgrrgdv'));//g:4

returnStrNum: function(str) {
    var strNum = {};
    for (var i = 0, l = str.length; i < l; i++) {
        strNum[str[i]] = (strNum[str[i]] + 1) || 1;
        if (strNum[str[i]] == 2) {
            return str[i]；
        }
    }
},
console.log(that.returnStrNum('sgdgfdgrrgdv'));//g

returnStrNum: function(str, subStr) {
    var sum = 0;
    var begin = 0;
    do {
        begin = str.indexOf(subStr, begin);
        if (begin != -1) {
            sum++;
            begin += subStr.length;
        }
    } while (begin != -1)
    return sum;
},
console.log(that.returnStrNum('sgdgfdgrrgdv', 'dg'));//2