2016 Update

Task/Find-the-missing-permutation/JavaScript/find-the-missing-permutation-4.js

@@ -0,0 +1,33 @@
+(() => {
+    'use strict';
+
+    // transpose :: [[a]] -> [[a]]
+    let transpose = xs =>
+        xs[0].map((_, iCol) => xs
+            .map((row) => row[iCol]));
+
+
+    let xs = 'ABCD CABD ACDB DACB BCDA ACBD ADCB CDAB' +
+        ' DABC BCAD CADB CDBA CBAD ABDC ADBC BDCA DCBA' +
+        ' BACD BADC BDAC CBDA DBCA DCAB'
+
+    return transpose(xs.split(' ')
+            .map(x => x.split('')))
+        .map(col => col.reduce((a, x) => ( // count of each character in each column
+            a[x] = (a[x] || 0) + 1,
+            a
+        ), {}))
+        .map(dct => { // character with frequency below mean of distribution ?
+            let ks = Object.keys(dct),
+                xs = ks.map(k => dct[k]),
+                mean = xs.reduce((a, b) => a + b, 0) / xs.length;
+
+            return ks.reduce(
+                (a, k) => a ? a : (dct[k] < mean ? k : undefined),
+                undefined
+            );
+        })
+        .join(''); // 4 chars as single string
+
+    // --> 'DBAC'
+})();