Initial data commit

Task/Quickselect-algorithm/JavaScript/quickselect-algorithm-1.js

@@ -0,0 +1,71 @@
+// this just helps make partition read better
+function swap(items, firstIndex, secondIndex) {
+  var temp = items[firstIndex];
+  items[firstIndex] = items[secondIndex];
+  items[secondIndex] = temp;
+};
+
+// many algorithms on this page violate
+// the constraint that partition operates in place
+function partition(array, from, to) {
+  // https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/random
+  var pivotIndex = getRandomInt(from, to),
+      pivot = array[pivotIndex];
+  swap(array, pivotIndex, to);
+  pivotIndex = from;
+
+  for(var i = from; i <= to; i++) {
+    if(array[i] < pivot) {
+      swap(array, pivotIndex, i);
+      pivotIndex++;
+    }
+  };
+  swap(array, pivotIndex, to);
+
+  return pivotIndex;
+};
+
+// later versions of JS have TCO so this is safe
+function quickselectRecursive(array, from, to, statistic) {
+  if(array.length === 0 || statistic > array.length - 1) {
+    return undefined;
+  };
+
+  var pivotIndex = partition(array, from, to);
+  if(pivotIndex === statistic) {
+    return array[pivotIndex];
+  } else if(pivotIndex < statistic) {
+    return quickselectRecursive(array, pivotIndex, to, statistic);
+  } else if(pivotIndex > statistic) {
+    return quickselectRecursive(array, from, pivotIndex, statistic);
+  }
+};
+
+function quickselectIterative(array, k) {
+  if(array.length === 0 || k > array.length - 1) {
+    return undefined;
+  };
+
+  var from = 0, to = array.length,
+      pivotIndex = partition(array, from, to);
+
+  while(pivotIndex !== k) {
+    pivotIndex = partition(array, from, to);
+    if(pivotIndex < k) {
+      from = pivotIndex;
+    } else if(pivotIndex > k) {
+      to = pivotIndex;
+    }
+  };
+
+  return array[pivotIndex];
+};
+
+KthElement = {
+  find: function(array, element) {
+    var k = element - 1;
+    return quickselectRecursive(array, 0, array.length, k);
+    // you can also try out the Iterative version
+    // return quickselectIterative(array, k);
+  }
+}