Google 面试题 第K小的数字 二分逼近&二分查找


Google 面试题 第K小的数字 二分逼近&二分查找
给定两个整型数组A和B。我们将A和B中的元素两两相加可以得到数组C。
譬如A为[1,2],B为[3,4].那么由A和B中的元素两两相加得到的数组C为[4,5,5,6]。
现在给你数组A和B,求由A和B两两相加得到的数组C中,第K小的数字。

那么得想想其他办法,比如说二分,但是该二分什么呢,一下子很难想到,一般情况下都是二分给定的数据中有的数字,但是这儿行不通。
后来想到了可以直接二分答案,尽管这个答案不在生成的数列中。
我们可以用二分逼近。
二分一个答案X
然后统计一下生成的序列中<=X的有多少个。
如果个数>=K,那么这是一个可能的解,记录一下。
然后我们往小的逼近。
如果<K说明当前的答案不够大。
要往大的逼近。

计算比X小的数字有多少个可以利用数组的单调性来做。
先对a,b数字从小到大排序。
然后枚举a中的元素a,统计一下b中有多少个和a加起来是<=X的。
a<=a[i+1]那么b中的符合的个数将会单调不增。
这样验证的复杂度就是O(n+m)了。
最后的总复杂度是O(log(2*10^9)*(n+m))
完美的复杂度

https://blog.csdn.net/u011846436/article/details/38468555
1.对两数组分别进行排序。
2.使用剪切法,如果a[i]+b[j]>val,则a[i]+b[j+1],a[i]+b[j+2]...a[i]+b[n]均大于val;反之,则说明数组a选取第i个元素作为左加数后,数组b中有j个元素可以作为右加数,能够使a[i]+b[0...j]<=val。
3.对i进行遍历,则可以确定值val在合并后的数组C中排名(很关键)。
4.采用二分法,求出中值mid在合并后的数组中排名,如果mid的排名>k,说明排第k位的数在mid左侧;否则,去右侧查找


05typedef long long lld;
06const lld M = 100001;
07lld arr1[M],arr2[M];
08lld m,n,k;
09
10//统计小于等于 mid 的个数
11lld cntMin(lld mid){
12    lld cnt = 0;
13    lld j = n-1;
14    for(int i=0; i<m; i++){
15        //由于已排序,可直接利用上一次结果
16        while(j >= 0 && arr1[i] + arr2[j] > mid) j--;
17        cnt += j+1;
18    }
19    return cnt;
20}
21
22lld low,high,mid;
23int main() {
24    freopen("in.txt""r", stdin);
25    while(scanf("%lld%lld%lld", &m,&n,&k) != EOF){
26        for(lld i=0; i<m; i++) scanf("%lld", &arr1[i]);
27        for(lld j=0; j<n; j++) scanf("%lld", &arr2[j]);
28        sort(arr1, arr1+m);
29        sort(arr2, arr2+n);
30         low  = arr1[0] + arr2[0];
31         high = arr1[m-1] + arr2[n-1];
32         lld ans;
33         //二分逼近
34        while(low <= high){
35             mid = (low+high)/2;
36            lld cnt = cntMin(mid);
37            if(cnt >= k){
38                ans = mid; //mid有可能是解
39                high = mid-1;
40            }else
41                low = mid+1;
42        }
43        printf("%lld\n",ans);
44
45    }
46    return 0;
47}

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