二分总结

二分查找基本模板：查询一个有序数列中等于target那个数字的下标（在这里如果有多个target会怎么样？）

{    while(low <= high){        int mid= high - (high - low)/2;        if( a[mid] < target )            low = mid+1;        else if( a[mid] > target)            high = mid-1;        else            return mid;    }}

一.在target为多个的时候，查询等于target的第一个数的下标

int Bsearch_1(int a[],int low,int high,int target){    while(low <= high){        int mid= (high + low)/2;        if( a[mid] < target )            low = mid+1;        else if( a[mid] > target)            high = mid-1;        else{            if(mid == 0 || a[mid]!=a[mid-1])                return mid;            else                high = mid - 1;        }    }}

二.在target为多个的时候，查询等于target的最后一个数的下标

int Bsearch_2(int a[],int low,int high,int target){    while(low <= high){        int mid= (high + low)/2;        if( a[mid] < target )            low = mid+1;        else if( a[mid] > target)            high = mid-1;        else{            if(mid == high || a[mid]!=a[mid+1])                return mid;            else                low = mid + 1; //        }    }}

三.查询大于target的第一个数的下标

int Bsearch_1(int a[],int low,int high,int target){    while(low <= high){        int mid= (high + low)/2;        if( a[mid] <= target )            low = mid+1;        else            high = mid-1;    }    return low;}

三.查询小于target的第一个数的下标

int Bsearch_4(int a[],int low,int high,int target){    int mid;    while(low <= high){        int mid= (high + low)/2;        if( a[mid] >= target )            high = mid-1;        else            low = mid+1;    }    return high;}

同三