Add solution 556

Author: vulgur

556_NextGreaterElement3.swift

@@ -0,0 +1,44 @@
+class Solution {
+    func nextGreaterElement(_ n: Int) -> Int {
+        if n <= 10 {
+            return -1
+        }
+        
+        var num = n
+        var digits = [Int]()
+        var maxDigit = -1
+        var lastDigit = -1
+        while num > 0 {
+            let digit = num % 10
+            lastDigit = digit
+            num = num / 10
+            digits.append(digit)
+            if digit >= maxDigit {
+                maxDigit = digit
+            } else {
+                break
+            }
+        }
+
+        digits = digits.sorted()
+        var index = 0
+        while index < digits.count && digits[index] <= lastDigit{
+            index += 1
+        }
+        if index == digits.count {
+            return -1
+        }
+        var tail = digits[index]
+        for i in 0..<digits.count {
+            num = num * 10
+            if i == index {
+                continue
+            } else {
+                tail = tail * 10 + digits[i]
+            }
+            
+        }
+        let result = num+tail
+        return result > n && result <= Int32.max ? result : -1
+    }
+}