线段树---初识

之前翻帖子有大概的听过这样一种数据结构，今天刚好刷题遇到了，记录一下自己写的吧！（没有加懒惰标记！！！）

class NumArray {
public:

    int tree[100000] = {0};
    int n = 0;
    void buildTree(vector<int> &num,int l,int r,int rt){
        if(l == r){
            tree[rt] = num[l-1];
            return;
        }

        int mid = (l+r)/2;
        buildTree(num,l,mid,rt*2);
        buildTree(num,mid+1,r,rt*2+1);
        tree[rt] = tree[rt*2]+tree[rt*2+1];
    }
    NumArray(vector<int>& nums) {
        if(nums.size()==0)return;
        n = nums.size();
        buildTree(nums,1,nums.size(),1);
    }

    void ud(int l,int r,int i,int val,int rt){
        if(l == r){
            int temp = tree[rt];
            //tree[rt] = val;
            while(rt > 0){
                tree[rt] = tree[rt]-temp+val;
                rt = rt/2;
            }
            return;
        }else{
            int mid = (l+r)/2;
            if(i <= mid) ud(l,mid,i,val,rt*2);
            else if(i > mid)ud(mid+1,r,i,val,rt*2+1);
        }
    }
    void update(int i, int val) {
        ud(1,n,i+1,val,1);
    }

    int sum(int l,int r,int i,int j,int rt){
        if(l == i&&r == j)return tree[rt];
        int mid = (l+r)/2;
        if(j <= mid) return sum(l,mid,i,j,rt*2);
        else if(i > mid) return sum(mid+1,r,i,j,rt*2+1);
        else{
            return sum(l,mid,i,mid,rt*2)+sum(mid+1,r,mid+1,j,rt*2+1);
        }
    }

    int sumRange(int i, int j) {
        return sum(1,n,i+1,j+1,1);
    }
};