Data commit

Task/N-queens-problem/Dart/n-queens-problem.dart

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+/**
+Return true if queen placement q[n] does not conflict with
+other queens q[0] through q[n-1]
+*/
+isConsistent(List q, int n) {
+  for (int i=0; i<n; i++) {
+    if (q[i] == q[n]) {
+      return false; // Same column
+    }
+
+    if ((q[i] - q[n]) == (n - i)) {
+      return false; // Same major diagonal
+    }
+
+    if ((q[n] - q[i]) == (n - i)) {
+      return false; // Same minor diagonal
+    }
+  }
+
+  return true;
+}
+
+/**
+Print out N-by-N placement of queens from permutation q in ASCII.
+*/
+printQueens(List q) {
+  int N = q.length;
+  for (int i=0; i<N; i++) {
+    StringBuffer sb = new StringBuffer();
+    for (int j=0; j<N; j++) {
+      if (q[i] == j) {
+        sb.write("Q ");
+      } else {
+        sb.write("* ");
+      }
+    }
+    print(sb.toString());
+  }
+  print("");
+}
+
+/**
+Try all permutations using backtracking
+*/
+enumerate(int N) {
+  var a = new List(N);
+  _enumerate(a, 0);
+}
+
+_enumerate(List q, int n) {
+  if (n == q.length) {
+    printQueens(q);
+  } else {
+    for (int i = 0; i < q.length; i++) {
+      q[n] = i;
+      if (isConsistent(q, n)){
+        _enumerate(q, n+1);
+      }
+    }
+  }
+}
+
+void main() {
+  enumerate(4);
+}