leecode刷题(2021-12-23)

1.两数之和

/**
 * @param {number[]} nums
 * @param {number} target
 * @return {number[]}
 */
var twoSum = function(nums, target) {
    for(let i=0;i<nums.length;i++){
        let rest = target - nums[i]
        nums[i] = NaN
        if(nums.includes(rest)){
            let index = nums.indexOf(rest)
            return [i,index]
        }
    }
};

 

2.回文数

/**
 * @param {number} x
 * @return {boolean}
 */
var isPalindrome = function(x) {
    if(x===0) return true
    if(x<0 || x%10===0) return false
    let str = x.toString()
    if(str.split('').reverse().join('')===str){
        return true
    }else{
        return false
    }
};

 

3.罗马数字转整形

/**
 * @param {string} s
 * @return {number}
 */
var romanToInt = function(s) {
    let total = 0
    let romanArr = s.split('')
    for (let i=0;i<romanArr.length;i++) {
        if(romanArr[i]+romanArr[i+1] === 'IV'){
            total += 4
            i++
            continue
        }
        if(romanArr[i]+romanArr[i+1] === 'IX'){
            total += 9
            i++
            continue
        }
        if(romanArr[i]+romanArr[i+1] === 'XL'){
            total += 40
            i++
            continue
        }
        if(romanArr[i]+romanArr[i+1] === 'XC'){
            total += 90
            i++
            continue
        }
        if(romanArr[i]+romanArr[i+1] === 'CD'){
            total += 400
            i++
            continue
        }
        if(romanArr[i]+romanArr[i+1] === 'CM'){
            total += 900
            i++
            continue
        }
        if(romanArr[i] === 'I'){
            total += 1
            continue
        }   
        if(romanArr[i] === 'V'){
            total += 5
            continue
        }  
        if(romanArr[i] === 'X')
        {
            total += 10
            continue
        }  
        if(romanArr[i] === 'L'){
            total += 50
            continue
        }  
        if(romanArr[i] === 'C'){
            total += 100
            continue
        }  
        if(romanArr[i] === 'D'){
            total += 500
            continue
        }  
        if(romanArr[i] === 'M'){
            total += 1000
            continue
        }  
    }
    return total
};

 

4.最长公共前缀

/**
 * @param {string[]} strs
 * @return {string}
 */
var longestCommonPrefix = function(strs) {
    let index = -1
    let minLength = 300
    let res = ""
    const newArr = []
    strs.forEach(item =>{
        if(minLength>item.length) minLength = item.length
        newArr.push(item.split(''))
    })
    for(let i=0;i<minLength;i++){
        if(newArr.every(item=>{return item[i] === newArr[0][i]})) {
            index = i
        }else{
            break
        }
    }
    if(index === -1){
        return ""
    }else{
        for(let j=0;j<=index;j++){
            res += newArr[0][j]
        }
        return res
    }
};

 

5.有效的括号

/**
 * @param {string} s
 * @return {boolean}
 */
var isValid = function(s) {
    const arr = s.split('')
    const stack = []
    let flag = true
    if(arr.length%2!==0){
        return false
    }
    arr.forEach(item =>{
        if(item==='(' || item==='[' || item ==='{'){
            stack.push(item)
        }else{
            if(item === ')'){
                if (stack.pop()!=='(') flag = false //这里用return不起作用，还待研究
            }
            if(item === ']'){
                if (stack.pop()!=='[') flag = false
            }
            if(item === '}'){
                if (stack.pop()!=='{') flag = false
            }
        }
    })
    if(stack.length===0){
        return true && flag
    }else{
        return false
    }

};

 

6.存在重复元素

/**
 * @param {number[]} nums
 * @return {boolean}
 */
var containsDuplicate = function(nums) {
   return nums.length === [...new Set(nums)].length ?  false :  true
};

 

7.最大子数组和

我的暴力解法超时，但也想记录一下，代码如下：

/**
 * @param {number[]} nums
 * @return {number}
 */
var maxSubArray = function(nums) {
  let max = -Infinity
  let temp = 0
  if(nums.length===1){
    return nums[0]
  }
  for(let i=0;i<nums.length;i++){
    temp =nums[i]
    if(max<temp){
      max = temp
    }
    for(let j=i+1;j<nums.length;j++){
      temp = temp+nums[j]
      if(max<temp){
        max = temp
      }else{
        continue
      }
    }
  }
  return max
};

动态规划方法：

/**
 * @param {number[]} nums
 * @return {number}
 */
var maxSubArray = function(nums) {
    let ans = nums[0];
    let sum = 0;
    for(let num of nums) {
        if(sum + num > num ){
            sum = sum + num;
        } else {
            sum = num;
        }
        ans = Math.max(ans, sum);
    };
    return ans;
};