js算法

1、js 统计一个字符串出现频率最高的字母/数字

let str = "asdfghjklaqwertyuiopiaia";
const strChar = (str) => {
  let string = [...str],
    maxValue = "",
    obj = {},
    max = 0;
  string.forEach((value) => {
    obj[value] = obj[value] === undefined ? 1 : obj[value] + 1;
    if (obj[value] > max) {
      max = obj[value];
      maxValue = value;
    }
  });
  return maxValue;
};
console.log(strChar(str)); // a

2、数组去重

2.1、forEach

let arr = ['1', '2', '3', '1', 'a', 'b', 'b']
const unique = arr => {
    let obj = {}
    arr.forEach(value => {
        obj[value] = 0
    })
    return Object.keys(obj)
}
console.log(unique(arr))  // ['1','2','3','a','b']

2.2、filter

let arr = ['1', '2', '3', '1', 'a', 'b', 'b']
const unique = arr => {
    return arr.filter((ele, index, array) => {
        return index === array.indexOf(ele)
    })
}
console.log(unique(arr))  // ['1','2','3','a','b']

2.3、set

let arr = ['1', '2', '3', '1', 'a', 'b', 'b']
const unique = arr => {
    return [...new Set(arr)]
}
console.log(unique(arr))  // ['1','2','3','a','b']

2.4、reduce

let arr = ['1', '2', '3', '1', 'a', 'b', 'b']
const unique = arr.reduce((map, item) => {
    map[item] = 0
    return map
}, {})
console.log(Object.keys(unique))  // ['1','2','3','a','b']

3、翻转字符串

let str ="Hello Dog";
const reverseString = str =>{
    return [...str].reverse().join("");
}
console.log(reverseString(str))   // goD olleH

4、数组中最大差值

4.1、forEach

let arr = [23, 4, 5, 2, 4, 5, 6, 6, 71, -3];
const difference = arr => {
    let min = arr[0],
        max = 0;
    arr.forEach(value => {
        if (value < min) min = value
        if (value > max) max = value
    })
    return max - min ;
}
console.log(difference(arr))  // 74

4.2、max、min

let arr = [23, 4, 5, 2, 4, 5, 6, 6, 71, -3];
const difference = arr => {
    let max = Math.max(...arr),
        min = Math.min(...arr);
    return max - min ;
}
console.log(difference(arr)) // 74

5、不借助临时变量，进行两个整数的交换

5.1、数组解构

let a = 2,
    b = 3;
    [b,a] = [a,b]
    console.log(a,b)   // 3 2

5.2、算术运算（加减）

输入a = 2,b = 3,输出 a = 3,b = 2

let a = 2,
    b = 3;
const swop = (a, b) => {
    b = b - a;
    a = a + b;
    b = a - b;
    return [a,b];
}
console.log(swop(2,3)) // [3,2]

5.3、逻辑运算（异或）

let a = 2,
    b = 3;
const swop = (a, b) => {
    a ^= b; //x先存x和y两者的信息
    b ^= a; //保持x不变，利用x异或反转y的原始值使其等于x的原始值
    a ^= b; //保持y不变，利用x异或反转y的原始值使其等于y的原始值
    return [a,b];
}
console.log(swop(2,3)) // [3,2]

6、排序 (从小到大)

6.1、冒泡排序

let arr = [43, 32, 1, 5, 9, 22];
const sort = arr => {
    arr.forEach((v, i) => {
        for (let j = i + 1; j < arr.length; j++) {
            if (arr[i] > arr[j]) {
                [arr[i],arr[j]] = [arr[j],arr[i]]
            }
        }
    })
    return arr
}
console.log(sort(arr))  // [1, 5, 9, 22, 32, 43]

// 数组分组求和
  let groupSum = {
    groupBy: function (array, callback) {
      return new Promise((resolve) => {
        let groups = {};
        array.forEach(item => {
          let group = JSON.stringify(callback(item));
          groups[group] = groups[group] || [];
          groups[group].push(item);
        });
        let res = Object.keys(groups).map(group => {
          return groups[group];
        });
        resolve(res)
      })
    },
    getSum: function (arr,bykey) {
      return new Promise((resolve) => {
        let res = this.groupBy(arr, function (item) {
          return item.shopId
        }).then(res => {
          console.log(res)
          let resultSum = res.map(item => {
            let sum = item.reduce((total, curr) => {
              return total + curr[bykey]
            }, 0);
            return sum
          })
          return resultSum
        })
        resolve(res)
      })
    }
  }
  let testArr = [
    { shopId: 1, shopName: '测试', money: 0, },
    { shopId: 2, shopName: '123', money: 1 },
    { shopId: 1, shopName: '测试', money: 2 },
    { shopId: 1, shopName: '测试', money: 3 },
    { shopId: 2, shopName: '123', money: 4 },
    { shopId: 1, shopName: '测试', money: 5 },
    { shopId: 2, shopName: '123', money: 6 },
  ]
  groupSum.getSum(testArr,'money').then(res => {
    console.log(res)
  }).catch(err => {
    console.log(err)
  })