java - printing optimal binary search tree in preorder dynamic programming algorith -


i have finished computing average cost of obst , know computed correctly. next task print tree in preorder. have attempt @ using recursion can't seem shake null pointer error.

here's code:

public class obst { static string[] keysa; static integer[][] root;  public static void main(string[] args) {          scanner sc = new scanner(system.in);         int tot = sc.nextint();         hashmap<string, double> hm = new hashmap<string, double>();          int uniqnum = 0;         string[] rawinput = new string[tot];          for(int i=0; i<tot; i++) {             string tmp1 = sc.next();             if(i==0) {                 hm.put(tmp1, 1.0);                 uniqnum += 1.0;             } else if( != 0) {                 if(!hm.containskey(tmp1)) {                     hm.put(tmp1, 1.0);                     uniqnum += 1.0;                 } else {                     double tmpfreq = 0.0;                     tmpfreq = hm.get(tmp1);                     hm.put(tmp1, (tmpfreq + 1.0));                 }             }            }         set<string> keys = hm.keyset();         keysa =  keys.toarray(new string[uniqnum]);         double[] freqsa = new double[uniqnum];         arrays.sort(keysa);         for(int i=0; i<uniqnum; i++) {             double tmp = 0.0;             string tmpk = keysa[i];             tmp = hm.get(tmpk);             tmp = tmp/tot;             freqsa[i] = tmp;         }         double[][] eee = new double[uniqnum+2][uniqnum+1];         double[][] www = new double[uniqnum+2][uniqnum+1];         //matrix store optimal structure         root = new integer[uniqnum+1][uniqnum+1];         for(int i=1; i<uniqnum+2; i++) {             eee[i][i-1] = 0.0;             www[i][i-1] = 0.0;         }         for(int l=1; l<uniqnum+1; l++) {             for(int i=1; i<=uniqnum-l+1; i++) {                 int j = + l - 1;                 eee[i][j] = double.max_value;                 www[i][j] = www[i][j-1] + freqsa[j-1];                 for(int r=i; r<=j; r++) {                     double t = eee[i][r-1] + eee[r+1][j] + www[i][j];                     if(t<eee[i][j]) {                         eee[i][j] = t;                         root[i][j] = r-1;                     }                 }             }         }         //total cost         system.out.println(eee[1][uniqnum]);             printtree(1,uniqnum-1,-1, ""); }   public static void printtree(int min, int max, int parent, string s) {     int r = root[min][max];     if(parent == -1 ) {         system.out.println(keysa[r] + " root");     } else if(min < parent) {         system.out.println(keysa[r] + " left child of " + s);     } else {         system.out.println(keysa[r] + " right child of " + s);     } if(min < max) {         printtree(min,r,r+1,keysa[r]);         printtree(r+1,max,r,keysa[r]);     } } }

my trouble in method print tree.

looks aren't checking bounds correctly. if there no left or right child, shouldn't printing side. make sure check r+1 within array size, , node exists there. same both left , right sides.


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