4.4力扣刷题记录

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• • • 1. 区域和检索 - 数组可修改--线段树

1. 区域和检索 - 数组可修改–线段树

题目地址
写了个线段树的轮子：线段树数据结构，数组seg_tree，构造函数build_tree，单点修改函数update_tree，区间查询函数query_tree。

b站线段树讲解+手撕代码

class NumArray {
    vector<int> nums;

public:
    NumArray(vector<int>& nums) {
        this->nums=nums;
        seg_tree.resize(pow(2, (int)ceil(log2(nums.size())) + 1));
        build_tree(0, 0, nums.size() - 1);
    }
    
    void update(int index, int val) {
        update_tree(0, 0, nums.size() - 1, index, val);
    }
    
    int sumRange(int left, int right) {
        return query_tree(0, 0, nums.size() - 1, left, right);
    }


private:
    vector<int> seg_tree;//线段树
    //变量名中有node的都是线段树的元素，否则是nums数组中的元素
    void build_tree(int node, int start, int end){//建树
        if(start == end){
            seg_tree[node] = nums[start];
            return;
        }
        int mid = (start + end) / 2;
        int left_node = node * 2 + 1;
        int right_node = node * 2 + 2;
        build_tree(left_node, start, mid);
        build_tree(right_node, mid + 1, end);
        seg_tree[node] = seg_tree[left_node] + seg_tree[right_node];
    }

    void update_tree(int node, int start, int end, int index, int val){//更新线段树
        if(start == end){
            nums[index] = val;
            seg_tree[node] = val;
            return;
        }
        int mid = (start + end) / 2;
        int left_node = node * 2 + 1;
        int right_node = node * 2 + 2;
        if(index <= mid)
            update_tree(left_node, start, mid, index, val);
        else
            update_tree(right_node, mid + 1, end, index, val);
        seg_tree[node] = seg_tree[left_node] + seg_tree[right_node];
    }

    int query_tree(int node, int start, int end, int left, int right){//查询区间和
        if(start > right || end < left){
            return 0;
        }
        else if(start >= left && end <= right){
            return seg_tree[node];
        }
        int mid = (start + end) / 2;
        int left_node = node * 2 + 1;
        int right_node = node * 2 + 2;
        int left_sum = query_tree(left_node, start, mid, left, right);
        int right_sum = query_tree(right_node, mid+1, end, left, right);
        return left_sum + right_sum;
    }
};