Add code for suffix array

Author: coderick14

suffix_array.cpp

@@ -0,0 +1,69 @@
+#include <bits/stdc++.h>
+#define ll long long
+using namespace std;
+
+struct ranking {
+    ll index;
+    ll first;
+    ll second;
+};
+
+bool comp(ranking a, ranking b) {
+    if (a.first == b.first) {
+        return a.second < b.second;
+    }
+    return a.first < b.first;
+}
+
+vector<ll> build_suffix_array(string s) {
+    const ll n = s.length();
+    const ll lgn = ceil(log2(n));
+
+    // vector to hold final suffix array result
+    vector<ll> sa(n);
+
+    // P[i][j] holds the ranking of the j-th suffix
+    // after comparing the first 2^i characters
+    // of that suffix
+    ll P[lgn][n];
+
+    // vector to store ranking tuples of suffixes
+    vector<ranking> ranks(n);
+    
+    ll i, j, step = 1;
+    for(j = 0; j < n; j++) {
+        P[0][j] = s[j] - 'a';
+    }
+
+    for(i = 1; i <= lgn; i++, step++) {
+        for(j = 0; j < n; j++) {
+            ranks[j].index = j;
+            ranks[j].first = P[i-1][j];
+            ranks[j].second = (j + pow(2,i-1) < n) ? P[i-1][j + (ll)(pow(2,i-1))] : -1;
+        }
+
+        sort(ranks.begin(), ranks.end(), comp);
+
+        for(j = 0; j < n; j++) {
+            P[i][ranks[j].index] = (j > 0 && ranks[j].first == ranks[j-1].first && ranks[j].second == ranks[j-1].second) ? P[i][ranks[j-1].index] : j;
+        }
+    }
+
+    step -= 1;
+
+    for(i = 0; i < n; i++) {
+        sa[P[step][i]] = i;
+    }
+
+    return sa;
+}
+
+int main() {
+    string s;
+    cin >> s;
+    vector<ll> sa = build_suffix_array(s);
+    for(ll i = 0; i < s.length(); i++) {
+        cout << i << " : " << sa[i] << endl;
+    }
+    return 0;
+}