Stop early in Bellman-Ford

This also eliminates the need for a separate extra iteration.

algorithms/graph/bellman_ford.js

@@ -39,31 +39,35 @@ var bellmanFord = function (graph, startNode) {
   var sourceDistance;
   var targetDistance;
 
-  for (var i = 0; i < adjacencyListSize - 1; i++) {
+  var iteration;
+  for (iteration = 0; iteration < adjacencyListSize; ++iteration) {
+    var somethingChanged = false;
+
     for (var j = 0; j < edgesSize; j++) {
       sourceDistance = minDistance[edges[j].source] + edges[j].weight;
       targetDistance = minDistance[edges[j].target];
 
       if (sourceDistance < targetDistance) {
+        somethingChanged = true;
         minDistance[edges[j].target] = sourceDistance;
         previousVertex[edges[j].target] = edges[j].source;
       }
     }
-  }
-
-  for (i = 0; i < edgesSize; i++) {
-    sourceDistance = minDistance[edges[i].source] + edges[i].weight;
-    targetDistance = minDistance[edges[i].target];
 
-    if (sourceDistance < targetDistance) {
-
-      // Empty 'distance' object indicates Negative-Weighted Cycle
-      return {
-        distance: {}
-      };
+    if (!somethingChanged) {
+      // Early stop.
+      break;
     }
   }
 
+  // If the loop did not break early, then there is a negative-weighted cycle.
+  if (iteration == adjacencyListSize) {
+    // Empty 'distance' object indicates Negative-Weighted Cycle
+    return {
+      distance: {}
+    };
+  }
+
   return {
     distance: minDistance,
     previous: previousVertex