js算法相关题目

异步方法并发数量的控制

如有异步方法

    function loadAsync(time) {
      return new Promise(resolve => {
        setTimeout(() => {
          resolve("执行" + time)
        }, time)
      })
    }

1. reduce+promise执行

    function limitLoad(loadFn, url, limitNum) {
      let totalUrl = [].concat(url);
      let res = [];
      let promiseQueue = totalUrl.splice(0, limitNum).map((item, index) => {
        return loadFn(item).then(item => {
          res.push(item);
          return index;
        })
      })

      totalUrl.reduce((last, current, nowIndex) => {
        return last.then(() => {
          return Promise.race(promiseQueue)
        }).then(fastIndex => {
          promiseQueue[fastIndex] = loadFn(current).then(item => {
            res.push(item);
            return nowIndex;
          })
        })
      }, Promise.resolve()).then(() => {
        Promise.all(promiseQueue).then(() => {
          console.log('异步队列顺序', url)
          console.log('执行顺序', res)
        })
      })
    }

2. 循环加递归

    function limitLoad2(loadFn, url, limitNum) {
      return new Promise(resolve => {
        const allUrl = url;
        const Length = allUrl.length;
        const res = [];
        let i = 0;
        const work = () => {
          while (allUrl.length && i < limitNum) {
            const useUrl = allUrl[0]
            allUrl.splice(0, 1)
            loadFn(useUrl).then(data => {
              res.push(data);
              i--;
              if (Length === res.length) {
                resolve(res);
              }
              work();
            });
            i++;
          }
        }
        work();
      })
    }

单次订阅发布

class Observer {
  constructor() {
    this.events_ = {}
  }
  onceEvent(event, callback) {
    var wrapFn = (...args) => {
      callback.apply(this, args);
    }
    this.on(event, wrapFn)
  }
  on(event, callback) {
    var callbacks = this.events_[event] || [];
    callbacks.push(callback);
    this.events_[event] = callbacks;
    return this;
  }
  emit(...args) {
    var event = args[0]
    var params = args.slice(1);
    var callbacks = this.events_[event]
    callbacks.forEach((fn, index) => {
      fn.apply(this, params);
      callbacks.splice(index, 1);
      this.events_[event] = callbacks;
      index--;
    })
    return this;
  }
}
var observer = new Observer();
observer.onceEvent("flush", (params) => console.log(params))
observer.emit("flush", "flush111");
observer.emit("flush", "flush222");

异步执行和js隐式转换

实现sum(1,2,3,4,5,6) = 21; sum中调用asyncAdd方法来相加

function asyncAdd(a, b, callback) {
  setTimeout(function () {
    callback(null, a + b)
  }, 1000)
}

• 三种方式实现:

1. 直接await (easy)
2. 并发执行 (mid)
3. js隐式转换 (hard)

async function sum(rest, type) {
  // 请在此处完善代码
  switch (type) {
    case 'easy':
      var result = rest.shift()
      for (var num of rest) {
        // 使用promise 获取相加结果
        result = await new Promise(resolve => {
          asyncAdd(result, num, (_, res) => {
            resolve(res)
          })
        })
      }
      // 返回执行结果
      return result;

    case 'mid':
      if (rest.length <= 1) {
        return rest[0] || 0
      }
      var promises = []
      // 遍历将数组里面的值两个两个的执行
      for (var i = 0; i < rest.length; i += 2) {
        promises.push(
          new Promise(resolve => {
            // 如果 rest[i+1] 是 undefined, 说明数组长度是奇数，这个是最后一个
            if (rest[i + 1] === undefined) {
              resolve(rest[i])
            } else {
              // 调用asyncAdd 进行计算
              asyncAdd(rest[i], rest[i + 1], (_, result) => {
                resolve(result)
              })
            }
          })
        )
      }
      // 获取第一次计算结果
      var result = await Promise.all(promises)
      // 然后将第一次获取到的结果即 [3,7,11] 再次调用 sum执行
      return await sum(...result);

    case 'hard':
      var result = 0
      // 隐氏类型转换， 对象 + 数字，会先调用对象的toString 方法
      var obj = {}
      obj.toString = function () {
        return result
      }
      var promises = []
      for (var num of rest) {
        promises.push(new Promise((resolve) => {
          asyncAdd(obj, num, (_, res) => {
            resolve(res)
          })
        }).then(res => {
          // 在这里将 result的值改变之后，obj.toString 的返回值就变了，这时候下一个setTimeout调用时就使用了新值
          result = res
        }))
      }
      await Promise.all(promises)
      return result
  }
}

消费金额price匹配拥有钞票最少的组合

function dfs_(tickets = [1,2,3,4,5,6,7,8,9], price = 39) {
  let tic = tickets.sort((a, b) => a - b).filter(a => a < price);
  let splurs = price;
  let res = [];
  let total = 0;

  while (splurs > 0 && !!tic.length) {
    let curentUse = 0;
    let curentTick = 0;

    curentTick = tic[tic.length - 1];
    curentUse = Math.floor(splurs / curentTick);

    if (curentUse > 0 && curentTick > 0) {
      splurs = splurs - curentUse * curentTick;
      res.push({
        tic: curentTick,
        count: curentUse,
      })
      total += curentUse;
    }

    if (curentTick) tic.splice(tic.length - 1, 1)

  }
  return splurs === 0 ? {
    total,
    res,
  } : "fail"
}

实现decodeString(“2[a3[b]]”) = abbbabbb;

   function decodeString(str) {
      var countStack = [];
      var itemStack = [];
      var multi = 0;
      var res = "";
      for (var a = 0; a < str.length; a++) {
        const cur = str.charAt(a);
        if (cur === "[") {
          countStack.push(multi);
          itemStack.push(res);
          res = "";
          multi = 0;
        } else if (cur === "]") {
          let count = countStack.pop();
          let temp = "";
          for (var i = 0; i < count; i++) {
            temp += res;
          }
          res = itemStack.pop() + temp;
        } else if (cur >= "0" && cur <= "9") {
          multi = multi * 10 + (cur - '0');
        } else {
          console.log(res, multi)
          res += cur;
        }
      }

      return res;
    }

    console.log(decodeString("2[3[a]2[bc]d]"))