哈希表

242. 有效的字母异位词

func isAnagram(s string, t string) bool {
    var m [26]int
    for _,v:=range s{
        m[v-'a']++
    }

    for _,k:=range t{
        m[k-'a']--
    }

    for _,w:=range m{
        if w!=0{
            return false
        }
    }

return true
}

349. 两个数组的交集

func intersection(nums1 []int, nums2 []int) []int {
    var arr []int
    h:=make(map[int]int,1)
    for _,k:=range nums1{
        h[k]++
    }

    for _,v:=range nums2{
        if h[v]>0{
            arr=append(arr,v)
            h[v]=0
        }
    }

    return arr
}

202. 快乐数

用哈希表来检测循环，出现出过的数就直接返回false

func isHappy(n int) bool {
h:=make(map[int]int,0)

for n!=1{
    if h[n]>0{
        return false
    }
     h[n]++ //要先保存在map中，再更新
n=getsum(n)

}
return true
}

func getsum(n int)int{
    sum:=0

for n>0{
    sum+=(n%10)*(n%10)
    n=n/10
}
    return sum
}

1. 两数之和

func twoSum(nums []int, target int) []int {

    h:=make(map[int]int,len(nums))

    for k,v:=range nums{
       if p,ok:=h[target-v];ok{
            return []int{k,p}
        }
        h[v]=k
    }
    return []int{}
}

454. 四数相加 II

func fourSumCount(nums1 []int, nums2 []int, nums3 []int, nums4 []int) int {
    h:=make(map[int]int)
    var t int
    for _,v1:=range nums1{
        for _,v2:=range nums2{
            h[v1+v2]++
        }
    }

    for _,v3:=range nums3{
        for _,v4:=range nums4{
            t+=h[0-v3-v4]
        }
    }

    return t
}

383. 赎金信

func canConstruct(ransomNote string, magazine string) bool {
    var mag [26]int

    for _,v:=range magazine{
        mag[v-'a']++
    }

    for _,k:=range ransomNote{
        if mag[k-'a']<=0{
            return false
        }else{
            mag[k-'a']--
        }
    }
return true
}

15. 三数之和

两层循环+双指针

func threeSum(nums []int) [][]int {
    var res [][]int

    sort.Ints(nums)
    for i:=0;i<len(nums)-2;i++{
        n1:=nums[i]
        if nums[i]>0{
            break
        }
        if i>0&&nums[i]==nums[i-1]{
            continue
        }

        left:=i+1
        right:=len(nums)-1

        for right>left{
            n2:=nums[left]
            n3:=nums[right]

            if n1+n2+n3==0{
                res=append(res,[]int{n1,n2,n3})
                  for right>left&&nums[left]==n2{
                      left++
                  }
                for right>left&&nums[right]==n3{
                    right--
                }
            }else if n1+n2+n3>0{
                right--
            }else if n1+n2+n3<0{
                left++
            }
        } 
    }
    return res
}

18. 四数之和

每层循环都要去重

func fourSum(nums []int, target int) [][]int {
   res:=[][]int{}
    if len(nums)<4{
        return res
    }
    sort.Ints(nums)
    for i:=0;i<len(nums)-3;i++{
              n1:=nums[i]
            if i>0&& n1 == nums[i-1] {
			    continue
		}
        for j:=i+1;j<len(nums)-2;j++{
                n2:=nums[j]
            if j>i+1&& n2 == nums[j-1] {
			    continue
		}
            l:=j+1
            r:=len(nums)-1
            for l<r{
                n3:=nums[l]
                n4:=nums[r]

                if n1+n2+n3+n4==target{
                    res=append(res,[]int{n1,n2,n3,n4})
                    for l<r&&nums[l+1]==nums[l]{
                        l++
                    }
                    for  l<r&&nums[r-1]==nums[r]{
                        r--
                    }
                         l++
                         r--
                }else if  n1+n2+n3+n4>target{
                    r--
                }else{
                    l++
                }
            }
        }
    }
    return res
}