Segment Tree（線段樹）

線段樹代碼模板：

public class NumArray {
	Node root;
	int[] nums;
    public NumArray(int[] nums) {
    	this.nums=nums;
        if(nums.length>0){
    	    root=build(root,0,nums.length-1);
        }
    }
    public Node build(Node root,int left,int right){
    	if(left==right){
    		return new Node(left,right,nums[left]);
    	}
    	int mid=(left+right)/2;
    	root=new Node(left,right,0);
    	root.l=build(root.l,left,mid);
    	root.r=build(root.r,mid+1,right);
    	root.sum=root.l.sum+root.r.sum;
    	return root;
    }
    public void update(int i, int val) {
    	update(root,i,val);
    }
    public void update(Node root,int i,int val){
    	if(root.left==root.right&&root.left==i){
    		root.sum=val;return;
    	}
    	int mid=(root.left+root.right)/2;
    	if(i<=mid) update(root.l,i,val);
    	else update(root.r,i,val);
    	root.sum=root.l.sum+root.r.sum;
    }
    public int sumRange(int i, int j) {
        return query(root,i,j);
    }
    public int query(Node root,int i,int j){
    	if(root.left==i&&root.right==j){
    		return root.sum;
    	}
    	int mid=(root.left+root.right)/2;
    	if(mid>=j) return query(root.l,i,j);
    	else if(mid<i){
    		return query(root.r,i,j);
    	}else{
    		return query(root.l,i,mid)+query(root.r,mid+1,j);
    	}
    }
}
class Node{
	int left,right;
	Node l,r;
	int sum;
	public Node(int left, int right, int sum) {
		this.left = left;
		this.right = right;
		this.sum = sum;
	}
}