Wavelet Trees | Introduction

11 Jun, 2021
A wavelet tree is a data structure that recursively partitions a stream into two parts until we’re left with homogeneous data. The name derives from an analogy with the wavelet transform for signals, which recursively decomposes a signal into low-frequency and high-frequency components. Wavelet trees can be used to answer range queries efficiently.Consider the problem to find number of elements in a range [L, R] of a given array A which are less than x. One way to solve this problem efficiently is using Persistent Segment Tree data structure. But we can also solve this easily using Wavelet Trees. Let us see how! 
Constructing Wavelet Trees
Every node in a wavelet tree is represented by an array which is the subsequence of original array and a range [L, R]. Here [L, R] is the range in which elements of array falls. That is, ‘R’ denotes maximum element in the array and ‘L’ denotes the smallest element. So, the root node will contain the original array in which elements are in range [L, R]. Now we will calculate the middle of the range [L, R] and stable partition the array in two halfs for the left and right childs. Therefore, the left child will contains elements that lies in range [L, mid] and right child will contain elements that lies in the range [mid+1, R]. Suppose we are given an array of integers. Now we compute the mid (Max + Min / 2) and form two children. Left Children: Integers less than/equal to Mid Right Children: Integers greater than Mid We recursively perform this operation until all node of similar elements are formed.Given array : 0 0 9 1 2 1 7 6 4 8 9 4 3 7 5 9 2 7 0 5 1 0  
To construct a Wavelet Tree, let us see what will we need to store at each node. So at each node of the tree, we will store two arrays say S[] and freq[]. The array S[] will be a subsequence of the original array A[] and the array freq[] will store the count of the elements that will go to left and right childs of the node. That is, freq[i] will denote the count of elements from the first i elements of S[] that will go to left child. Therefore, count of elements that will go to right child can be easily calculated as (i – freq[i]).Below example shows how to maintain freq[] array:  
Array : 1 5 2 6 4 4
Mid = (1 + 6) / 2 = 3
Left Child : 1 2
Right Child : 5 6 4 4
To maintain frequency array, we will check if the element is less than Mid or not. If yes, then we will add 1 to last element of frequency array, else 0 and push back again.For, above array : Freq array :{1, 1, 2, 2, 2, 2}It implies 1 element will go to left child of this node from index 1 and 2, and 2 elements will go to left child from indices 3 to 6. This can be easily depicted from the above given array. To compute the number of elements moving to right subtree, we subtract freq[i] from i.  
From index 1, 0 elements go to right subtree.
From index 2, 1 element go to right subtree.
From index 3, 1 element go to right subtree.
From index 4, 2 elements go to right subtree.
From index 5, 3 elements go to right subtree.
From index 6, 4 elements go to right subtree.
We can use the stable_partition function and lambda expression in C++ STL to easily stable partition the array around a pivot without distorting the order of elements in original sequence. It is highly recommended to go through the stable_partition and lambda expression articles before moving onto implementation. Below is the implementation of construction of Wavelet Trees:  
CPP
// CPP code to implement wavelet trees#include <bits/stdc++.h>using namespace std;#define N 100000 // Given arrayint arr[N]; // wavelet tree classclass wavelet_tree {public:     // Range to elements    int low, high;     // Left and Right children    wavelet_tree* l, *r;     vector<int> freq;     // Default constructor    // Array is in range [x, y]    // Indices are in range [from, to]    wavelet_tree(int* from, int* to, int x, int y)    {        // Initialising low and high        low = x, high = y;         // Array is of 0 length        if (from >= to)            return;         // Array is homogeneous        // Example : 1 1 1 1 1        if (high == low) {             // Assigning storage to freq array            freq.reserve(to - from + 1);             // Initialising the Freq array            freq.push_back(0);             // Assigning values            for (auto it = from; it != to; it++)                 // freq will be increasing as there'll                // be no further sub-tree                freq.push_back(freq.back() + 1);                         return;        }         // Computing mid        int mid = (low + high) / 2;         // Lambda function to check if a number is        // less than or equal to mid        auto lessThanMid = [mid](int x) {            return x <= mid;        };         // Assigning storage to freq array        freq.reserve(to - from + 1);         // Initialising the freq array        freq.push_back(0);         // Assigning value to freq array        for (auto it = from; it != to; it++)             // If lessThanMid returns 1(true), we add            // 1 to previous entry. Otherwise, we add            // 0 (element goes to right sub-tree)            freq.push_back(freq.back() + lessThanMid(*it));         // std::stable_partition partitions the array w.r.t Mid        auto pivot = stable_partition(from, to, lessThanMid);         // Left sub-tree's object        l = new wavelet_tree(from, pivot, low, mid);         // Right sub-tree's object        r = new wavelet_tree(pivot, to, mid + 1, high);    }}; // Driver codeint main(){    int size = 5, high = INT_MIN;       int arr[] = {1 , 2, 3, 4, 5};    for (int i = 0; i < size; i++)        high = max(high, arr[i]);        // Object of class wavelet tree    wavelet_tree obj(arr, arr + size, 1, high);     return 0;}
Height of the tree: O(log(max(A)) , where max(A) is the maximum element in the array A[]. 
Querying in Wavelet Trees
We have already constructed our wavelet tree for the given array. Now we will move on to our problem to calculate number of elements less than or equal to x in range [ L,R ] in the given array. So, for each node we have a subsequence of original array, lowest and highest values present in the array and count of elements in left and right child.Now,  
If high <= x, 
   we return R - L + 1. 
i.e. all the elements in the current range is less than x.
Otherwise, We will use variable LtCount = freq[ L-1 ] (i.e. elements going to left sub-tree from L-1) , RtCount = freq[ R ] (i.e. elements going to right sub-tree from R) Now, we recursively call and add the return values of :  
left sub-tree with range[ LtCount + 1, RtCount ] and, 
right sub-tree with range[ L - Ltcount,R - RtCount ]
Below is the implementation in C++:  
CPP
// CPP program for querying in// wavelet tree Data Structure#include <bits/stdc++.h>using namespace std;#define N 100000 // Given Arrayint arr[N]; // wavelet tree classclass wavelet_tree {public:    // Range to elements    int low, high;     // Left and Right child    wavelet_tree* l, *r;     vector<int> freq;     // Default constructor    // Array is in range [x, y]    // Indices are in range [from, to]    wavelet_tree(int* from, int* to, int x, int y)    {        // Initialising low and high        low = x, high = y;         // Array is of 0 length        if (from >= to)            return;         // Array is homogeneous        // Example : 1 1 1 1 1        if (high == low) {            // Assigning storage to freq array            freq.reserve(to - from + 1);             // Initialising the Freq array            freq.push_back(0);             // Assigning values            for (auto it = from; it != to; it++)                             // freq will be increasing as there'll                // be no further sub-tree                freq.push_back(freq.back() + 1);                         return;        }         // Computing mid        int mid = (low + high) / 2;         // Lambda function to check if a number        // is less than or equal to mid        auto lessThanMid = [mid](int x) {            return x <= mid;        };         // Assigning storage to freq array        freq.reserve(to - from + 1);         // Initialising the freq array        freq.push_back(0);         // Assigning value to freq array        for (auto it = from; it != to; it++)             // If lessThanMid returns 1(true), we add            // 1 to previous entry. Otherwise, we add 0            // (element goes to right sub-tree)            freq.push_back(freq.back() + lessThanMid(*it));                // std::stable_partition partitions the array w.r.t Mid        auto pivot = stable_partition(from, to, lessThanMid);         // Left sub-tree's object        l = new wavelet_tree(from, pivot, low, mid);         // Right sub-tree's object        r = new wavelet_tree(pivot, to, mid + 1, high);    }     // Count of numbers in range[L..R] less than    // or equal to k    int kOrLess(int l, int r, int k)    {        // No elements int range is less than k        if (l > r or k < low)            return 0;         // All elements in the range are less than k        if (high <= k)            return r - l + 1;         // Computing LtCount and RtCount        int LtCount = freq[l - 1];        int RtCount = freq[r];         // Answer is (no. of element <= k) in        // left + (those <= k) in right        return (this->l->kOrLess(LtCount + 1, RtCount, k) +             this->r->kOrLess(l - LtCount, r - RtCount, k));    } }; // Driver codeint main(){    int size = 5, high = INT_MIN;           int arr[] = {1, 2, 3, 4, 5};            // Array : 1 2 3 4 5    for (int i = 0; i < size; i++)            high = max(high, arr[i]);     // Object of class wavelet tree    wavelet_tree obj(arr, arr + size, 1, high);     // count of elements less than 2 in range [1,3]    cout << obj.kOrLess(0, 3, 2) << '\n';     return 0;}
Output :  
2
Time Complexity: O(log(max(A)) , where max(A) is the maximum element in the array A[]. In this post we have discussed about a single problem on range queries without update. In further we will be discussing on range updates also.References :  
https://users.dcc.uchile.cl/~jperez/papers/ioiconf16.pdf
https://en.wikipedia.org/wiki/Wavelet_Tree
https://www.youtube.com/watch?v=K7tju9j7UWU
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