线段树---数据结构学习

线段树的教程可以参照线段树
这里推荐 https://oi-wiki.org/
这个网站，数据结构讲的非常透。
线段树学了很多次忘了很多次，这次打算记录一下以后方便回顾(leetcode这类题遇见的不算特别多)。
样板例题 leltcode-307

#题目样板
class NumArray {
    private int[] segmentTree;
    private int n;

    public NumArray(int[] nums) {
        n = nums.length;
        segmentTree = new int[nums.length * 4];
        build(0, 0, n - 1, nums);
    }

    public void update(int index, int val) {
        change(index, val, 0, 0, n - 1);
    }

    public int sumRange(int left, int right) {
        return range(left, right, 0, 0, n - 1);
    }

    private void build(int node, int s, int e, int[] nums) {
        if (s == e) {
            segmentTree[node] = nums[s];
            return;
        }
        int m = s + (e - s) / 2;
        build(node * 2 + 1, s, m, nums);
        build(node * 2 + 2, m + 1, e, nums);
        segmentTree[node] = segmentTree[node * 2 + 1] + segmentTree[node * 2 + 2];
    }

    private void change(int index, int val, int node, int s, int e) {
        if (s == e) {
            segmentTree[node] = val;
            return;
        }
        int m = s + (e - s) / 2;
        if (index <= m) {
            change(index, val, node * 2 + 1, s, m);
        } else {
            change(index, val, node * 2 + 2, m + 1, e);
        }
        segmentTree[node] = segmentTree[node * 2 + 1] + segmentTree[node * 2 + 2];
    }

    private int range(int left, int right, int node, int s, int e) {
        if (left == s && right == e) {
            return segmentTree[node];
        }
        int m = s + (e - s) / 2;
        if (right <= m) {
            return range(left, right, node * 2 + 1, s, m);
        } else if (left > m) {
            return range(left, right, node * 2 + 2, m + 1, e);
        } else {
            return range(left, m, node * 2 + 1, s, m) + range(m + 1, right, node * 2 + 2, m + 1, e);
        }
    }
}