树状数组求逆序对

 

 

class Solution {
    public int reversePairs(int[] nums) {
         int l = nums.length;
         int []tmp  = new int[l];//l就可以
         System.arraycopy(nums,0,tmp,0,l);
         Arrays.sort(tmp);
           //离散化
         for(int i=0;i<l;i++){
             nums[i]  = Arrays.binarySearch(tmp,nums[i])+1;
         }
         int ret  =0;
         BIT bit  =new BIT(l);
         for(int i=0;i<l;i++){
                 bit.update(nums[i],1);
                 ret+=(bit.getsum(l)-bit.getsum(nums[i]));
         }
         return ret;
    }
}

class BIT {
   private int n;
   private int[]c  ;
   public BIT(int n){
       this.n  = n;
       this.c  =new int[n+1];
   }
   int lowbit(int x){
       return x&(-x);
   }
   void update(int i,int num){
       while(i<=n){
           c[i]+=num;
           i+=lowbit(i);
       }
   }
   int getsum(int i){
       int sum  =0;
       while(i>0){
           sum+=c[i];
           i-=lowbit(i);
       }
       return sum;
   }
}