[LeetCode] Add January 31.

solved/LeetCode/Challenges/2020/January/31.kt

@@ -0,0 +1,38 @@
+class Solution {
+  fun nextPermutation(nums: IntArray): Unit {
+    if (nums.size <= 1) {
+      return
+    }
+    var last = nums.size - 2
+    while (last >= 0) {
+      if (nums[last + 1] > nums[last]) {
+        var right = last + 1
+        for (i in last + 1 until nums.size) {
+          if (nums[i] > nums[last]) {
+            right = i
+          }
+        }
+        nums[right] = nums[last]. also {
+          nums[last] = nums[right]
+        } 
+        break
+      }
+      last--
+    }
+    println(last)
+    nums.reverse(last + 1, nums.size)
+  }
+}
+
+
+fun IntArray.reverse(fromIndex: Int, toIndex: Int) {
+  var left = fromIndex
+  var right = toIndex -1
+  while (left < right) {
+    this[left] = this[right].also {
+      this[right] = this[left]
+    }
+    left++
+    right--
+  }
+}