We have discussed recursive segment tree implementation. In this post, iterative implementation is discussed.
Let us consider the following problem understand Segment Trees.
We have an array arr[0 . . . n-1]. We should be able to
1 Find the minimum of elements from index l to r where 0 <= l <= r <= n-1
2 Change value of a specified element of the array to a new value x. We need to do arr[i] = x where 0 <= i <= n-1.
Examples:
Input : 2, 6, 7, 5, 18, 86, 54, 2
minimum(2, 7)
update(3, 4)
minimum(2, 6)
Output : Minimum in range 2 to 7 is 2.
Minimum in range 2 to 6 is 4.
The iterative version of the segment tree basically uses the fact, that for an index i, left child = 2 * i and right child = 2 * i + 1 in the tree. The parent for an index i in the segment tree array can be found by parent = i / 2. Thus we can easily travel up and down through the levels of the tree one by one. At first we compute the minimum in the ranges while constructing the tree starting from the leaf nodes and climbing up through the levels one by one. We use the same concept while processing the queries for finding the minimum in a range. Since there are (log n) levels in the worst case, so querying takes log n time. For update of a particular index to a given value we start updating the segment tree starting from the leaf nodes and update all those nodes which are affected by the updation of the current node by gradually moving up through the levels at every iteration. Updation also takes log n time because there we have to update all the levels starting from the leaf node where we update the exact value at the exact index given by the user.
// CPP Program to implement iterative segment // tree. #include <bits/stdc++.h> #define ll long long using namespace std;
void construct_segment_tree(vector<int>& segtree,
vector<int> &a, int n)
{ // assign values to leaves of the segment tree
for (int i = 0; i < n; i++)
segtree[n + i] = a[i];
/* assign values to internal nodes
to compute minimum in a given range */
for (int i = n - 1; i >= 1; i--)
segtree[i] = min(segtree[2 * i],
segtree[2 * i + 1]);
} void update(vector<int>& segtree, int pos, int value,
int n)
{ // change the index to leaf node first
pos += n;
// update the value at the leaf node
// at the exact index
segtree[pos] = value;
while (pos > 1) {
// move up one level at a time in the tree
pos >>= 1;
// update the values in the nodes in
// the next higher level
segtree[pos] = min(segtree[2 * pos],
segtree[2 * pos + 1]);
}
} int range_query(vector<int>& segtree, int left, int right, int n)
{ /* Basically the left and right indices will move
towards right and left respectively and with
every each next higher level and compute the
minimum at each height. */
// change the index to leaf node first
left += n;
right += n;
// initialize minimum to a very high value
int mi = (int)1e9;
while (left < right) {
// if left index in odd
if (left & 1) {
mi = min(mi, segtree[left]);
// make left index even
left++;
}
// if right index in odd
if (right & 1) {
// make right index even
right--;
mi = min(mi, segtree[right]);
}
// move to the next higher level
left /= 2;
right /= 2;
}
return mi;
} // Driver code int main()
{ vector<int> a = { 2, 6, 10, 4, 7, 28, 9, 11, 6, 33 };
int n = a.size();
/* Construct the segment tree by assigning
the values to the internal nodes*/
vector<int> segtree(2 * n);
construct_segment_tree(segtree, a, n);
// compute minimum in the range left to right
int left = 0, right = 5;
cout << "Minimum in range " << left << " to " << right << " is "<< range_query(segtree, left,
right + 1, n) << "\n";
// update the value of index 3 to 1
int index = 3, value = 1;
// a[3] = 1;
// Contents of array : {2, 6, 10, 1, 7, 28, 9, 11, 6, 33}
update(segtree, index, value, n); // point update
// compute minimum in the range left to right
left = 2, right = 6;
cout << "Minimum in range " << left << " to "
<< right << " is " << range_query(segtree,
left, right + 1, n) << "\n";
return 0;
} |
Minimum in range 0 to 5 is 2 Minimum in range 2 to 6 is 1
Time Complexity(n log n)
Auxiliary Space (n)
Recommended Posts:
- Iterative Segment Tree (Range Maximum Query with Node Update)
- Segment Tree | Set 2 (Range Minimum Query)
- Segment Tree | Set 2 (Range Maximum Query with Node Update)
- Segment Tree | Set 3 (XOR of given range)
- Segment Tree | Set 1 (Sum of given range)
- Segment Tree | (XOR of a given range )
- Range Minimum Query (Square Root Decomposition and Sparse Table)
- Overview of Data Structures | Set 3 (Graph, Trie, Segment Tree and Suffix Tree)
- Cartesian tree from inorder traversal | Segment Tree
- Two equal sum segment range queries
- Range sum query using Sparse Table
- Range query for count of set bits
- Segment Trees | (Product of given Range Modulo m)
- Range query for Largest Sum Contiguous Subarray
- Range and Update Query for Chessboard Pieces
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