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_307_2.java
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_307_2.java
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/**
* LeetCode 307 - Range Sum Query - Mutable
*
* Segment Tree
* O(log n) update/query
*/
public class _307_2 {
int[] a;
int[] tree;
private void build(int i, int left, int right) {
if (left == right) tree[i] = a[left];
else {
int mid = (left + right) / 2;
build(2 * i, left, mid);
build(2 * i + 1, mid + 1, right);
tree[i] = tree[2 * i] + tree[2 * i + 1];
}
}
private void update(int i, int left, int right, int index, int val) {
if (left == index && right == index) tree[i] = val;
else if (left > index || right < index) return;
else {
int mid = (left + right) / 2;
update(2 * i, left, mid, index, val);
update(2 * i + 1, mid + 1, right, index, val);
tree[i] = tree[2 * i] + tree[2 * i + 1];
}
}
private int query(int i, int left, int right, int begin, int end) {
if (left > end || right < begin) return 0;
else if (begin <= left && right <= end) return tree[i];
else {
int mid = (left + right) / 2;
return query(2 * i, left, mid, begin, end) + query(2 * i + 1, mid + 1, right, begin, end);
}
}
public _307_2(int[] nums) {
a = nums;
tree = new int[4 * a.length + 1];
if (nums.length > 0) build(1, 0, nums.length - 1);
}
void update(int i, int val) {
a[i] = val;
update(1, 0, a.length - 1, i, val);
}
public int sumRange(int i, int j) {
return query(1, 0, a.length - 1, i, j);
}
}