線段樹的教程可以參照線段樹
這里推薦 https://oi-wiki.org/
這個網站，數據結構講的非常透。
線段樹學了很多次忘了很多次，這次打算記錄一下以后方便回顧(leetcode這類題遇見的不算特別多)。
樣板例題 leltcode-307

#題目樣板
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);}}
}