二分边界防止循环

数的范围为例

#include<iostream> #include<algorithm> #include<vector> using namespace std; int main(){ int n,q; scanf("%d%d",&n,&q); vector<int> a(n); for(int i=0;i<n;i++) scanf("%d",&a[i]); while(q--){ int k; scanf("%d",&k); int l=0,r=n-1; while(l<r){ int mid=l+r>>1; if(a[mid]>=k){ r=mid; } else{ l=mid+1; } } if(a[l]==k) printf("%d ",l); else{ printf("-1 -1\n");//未找到 continue; } int l2=0,r2=n-1; while(l2<r2){ int mid = l2 + r2 + 1 >> 1; if(a[mid]<=k){ l2=mid; } else{ r2=mid-1; } } printf("%d \n",l2); } }