01-复杂度3 二分查找 陈越、何钦铭数据结构

题目

可以满分通过的答案：
`
Position BinarySearch(List L, ElementType X){
Position left = 1;
Position right = L->Last;

while(left<=right){
Position center = (left+right)/2;
if(L->Data[center]>X){
right = center-1;
}else if(L->Data[center]<X){
left = center+1;
}else{
return center;
}
}
return NotFound;
}`

期间我发现，若利用递归，处理不当存在超时现象：

`Position search(ElementType arr[], ElementType X, Position left, Position right)
{
if(right == left){
if(arr[right] != X){
return NotFound;
}else{
return right;
}
}
Position center = (left+right)/2;
if(arr[center] > X){
search(arr,X,left,center);
}else if(arr[center] < X){
search(arr,X,center,right);
}else{
return center;
}
}

Position BinarySearch(List L, ElementType X){
return search(L->Data,X,1,L->Last);
}
`

`
Position search(ElementType arr[], ElementType X, Position left, Position right)
{
if(right == left){
if(arr[right] != X){
return NotFound;
}else{
return right;
}
}
Position center = (left+right)/2;
if(arr[center] > X){
search(arr,X,left,center-1);
}else if(arr[center] < X){
search(arr,X,center+1,right);
}else{
return center;
}
}

Position BinarySearch(List L, ElementType X){
return search(L->Data,X,1,L->Last);
}
`

递归改用这样即可不超时：
`
Position search(ElementType arr[], ElementType X, Position left, Position right)
{
if(right<left){
return NotFound;
}
Position center = (left+right)/2;
if(arr[center] > X){
search(arr,X,left,center-1);
}else if(arr[center] < X){
search(arr,X,center+1,right);
}else{
return center;
}
}

Position BinarySearch(List L, ElementType X){
return search(L->Data,X,1,L->Last);
}
`