线段树求对应数组区间的大小

构建线段树用了分治的思想

 

线段树查询数组中值的大小

//求数组对应区间值的大小
#include<bits/stdc++.h>
int const maxsize = 1000;
void build_tree(int arr[],int tree[],int node,int start,int end){ //创建线段树 
	if(start==end){
		tree[node]=arr[start];
	}
	
	else{
		int mid = (start+end)/2;
		int node_left=node*2+1;
		int node_right=node*2+2;
		build_tree(arr,tree,node_left,start,mid);
		build_tree(arr,tree,node_right,mid+1,end);
		tree[node]=tree[node_left]+tree[node_right];
	}
}
void updata_tree(int arr[],int tree[],int node,int start,int end,int index,int val){ //修改arr数组 重构线段树 
	if(end==start){ //找到了arr数组对应线段树最低层需要修改的值 
		tree[node] = val;
		arr[index] =val;
	}
	else
	{
		int mid = (start+end)/2;
		int node_left=node*2+1;
		int node_right=node*2+2;
		if(index<=mid){ //说明修改的点在线段树的左边 
		updata_tree(arr,tree,node_left,start,mid,index,val);
		}else{
		updata_tree(arr,tree,node_right,mid+1,end,index,val);
		}
		tree[node]=tree[node_left]+tree[node_right];//重构线段树	
	}
	
}
int query_tree(int arr[],int tree[],int node,int start,int end,int left,int right){
	
	if(left>end||right<start){ //递归返回条件 
		return 0;
	}else if(left<=start && end<=right){ //这个位置逻辑不要搞错了 当end start 在搜索区间的条件 可以直接返回了 
		return tree[node];
	}else if(start==end){  //此步可以省略，前一步已经包含了 
		return tree[node];
	}
	else{
		int mid=(start+end)/2;
		int node_left = node*2+1;
		int node_right = node*2+2;
		int sum_left=query_tree(arr,tree,node_left,start,mid,left,right);
		int sum_right=query_tree(arr,tree,node_right,mid+1,end,left,right);
		return sum_left+sum_right;	
	}
}
int main(){
	int arr[6]={1,3,5,7,9,11};
	int tree[maxsize]={0};
	int size = 6;
	build_tree(arr,tree,0,0,size-1);
//	for(int i=0;i<15;i++){
//		printf("%d\n",tree[i]);
//	}
//	updata_tree(arr,tree,0,0,size-1,4,6);
//	for(int i=0;i<15;i++){
//		printf("%d\n",tree[i]);
//	}
	int sum=query_tree(arr,tree,0,0,size-1,3,5);
	printf("%d\n",sum);
	return 0;
}