[leetcode]二分查找总结

Search for a Range

class Solution {
public int[] searchRange(int[] A, int target) {
int ans[]=new int[2];
int a= bserch(A,target);
if(a==-1) {ans[0]=-1;ans[1]=-1; return ans;}
//left
int b=a-1;
while(b>=0&&A[b]==target) b--;
ans[0]=b+1;
b=a+1;

while(b<=A.length-1&&A[b]==target) b++;
ans[1]=b-1;
return ans;

}

public  int bserch(int A[],int target)
{
int low=0;
int end=A.length-1;
while(low<=end)
{
int mid=(low+end)>>1;
if(A[mid]==target) return mid;
else if(A[mid]<target)  low=mid+1;
else   end=mid-1;

}

return -1;
}

}

class Solution {
public int[] searchRange(int[] A, int target) {
int ans[]=new int[2];
int a= bserch(A,target);
if(a==-1) {ans[0]=-1;ans[1]=-1; return ans;}
//left
int b=a-1;
while(b>=0&&A[b]==target) b--;
ans[0]=b+1;
b=a+1;

while(b<=A.length-1&&A[b]==target) b++;
ans[1]=b-1;
return ans;

}
//其实难点就是A[mid]==target，如何调整low和high
public  int bserch(int A[],int target)
{
int low=0;
int end=A.length-1;
while(low<=end)
{
int mid=(low+end)>>1;
if(A[mid]==target) return mid;
else if(A[mid]<target)  low=mid+1;
else   end=mid-1;

}

return -1;
}
//第一个大于target的
public int  upperbound(int A[],int target) // the first one larger than target
{
int low=0;
int end=A.length-1;
while(low<=end)
{
int mid=(low+end)>>1;
if(A[mid]>target) end=mid-1;
else low=mid+1;

}
return low;
}
public int  lowbound(int A[],int target) // the first one larger or equal  target
{
int low=0;
int end=A.length-1;
while(low<=end)
{
int mid=(low+end)>>1;
if(A[mid]>=target) end=mid-1;
else low=mid+1;

}
return low;
}

}