81 lines
2.6 KiB
Java
81 lines
2.6 KiB
Java
package DataStructures.Trees;
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public class SegmentTree {
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private int seg_t[];
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private int n;
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private int arr[];
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/* Constructor which takes the size of the array and the array as a parameter*/
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public SegmentTree(int n, int arr[]) {
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this.n = n;
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int x = (int) (Math.ceil(Math.log(n) / Math.log(2)));
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int seg_size = 2 * (int) Math.pow(2, x) - 1;
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this.seg_t = new int[seg_size];
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this.arr = arr;
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this.n = n;
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constructTree(arr, 0, n - 1, 0);
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}
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/* A function which will create the segment tree*/
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public int constructTree(int[] arr, int start, int end, int index) {
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if (start == end) {
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this.seg_t[index] = arr[start];
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return arr[start];
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}
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int mid = start + (end - start) / 2;
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this.seg_t[index] = constructTree(arr, start, mid, index*2 + 1) +
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constructTree(arr, mid + 1, end, index*2 + 2);
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return this.seg_t[index];
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}
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/* A function which will update the value at a index i. This will be called by the
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update function internally*/
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private void updateTree(int start, int end, int index, int diff, int seg_index) {
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if (index < start || index > end) {
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return;
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}
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this.seg_t[seg_index] += diff;
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if (start != end) {
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int mid = start + (end - start) / 2;
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updateTree(start, mid, index, diff, seg_index*2 + 1);
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updateTree(mid + 1, end, index, diff, seg_index*2 + 2);
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}
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}
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/* A function to update the value at a particular index*/
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public void update(int index, int value) {
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if (index < 0 || index > n) {
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return;
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}
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int diff = value - arr[index];
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arr[index] = value;
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updateTree(0, n - 1, index, diff, 0);
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}
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/* A function to get the sum of the elements from index l to index r. This will be called internally*/
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private int getSumTree(int start, int end, int q_start, int q_end, int seg_index) {
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if (q_start <= start && q_end >= end) {
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return this.seg_t[seg_index];
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}
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if (q_start > end || q_end < start) {
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return 0;
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}
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int mid = start + (end - start)/2;
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return getSumTree(start, mid, q_start, q_end, seg_index*2 + 1) + getSumTree(mid + 1, end, q_start, q_end, seg_index*2 + 2);
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}
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/* A function to query the sum of the subarray [start...end]*/
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public int getSum(int start, int end) {
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if (start < 0 || end > n || start > end) {
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return 0;
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}
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return getSumTree(0, n-1, start, end, 0);
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}
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} |