Create 279. Perfect Squares.md

Author: TBXark

279. Perfect Squares.md

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+# 100. Perfect Squares
+
+### 2020-07-30
+
+Given a positive integer n, find the least number of perfect square numbers (for example, `1, 4, 9, 16, ...`) which sum to n.
+
+Example 1:
+```
+Input: n = 12
+Output: 3 
+Explanation: 12 = 4 + 4 + 4.
+```
+
+Example 2:
+```
+Input: n = 13
+Output: 2
+Explanation: 13 = 4 + 9.
+```
+
+# Solution
+
+```swift
+class Solution {
+    
+    var cache = [Int: Int]()
+    var perfectSquare = [Int]()
+    
+    func findSum(count: Int, num: Int) -> Bool {
+        switch count {
+        case 0:
+            return false
+        case 1:
+            guard let c = cache[num] else {
+                return false
+            }
+            return c == 1
+        default:
+            if cache[num] != nil {
+                return true
+            }
+            for sn in perfectSquare {
+                if findSum(count: count - 1, num: num - sn) {
+                    cache[num] = count
+                    return true
+                }
+            }
+            return false
+        }
+    }
+    
+    func isPerfectSquare(n: Int) -> Bool {
+        let sqN = Int(sqrt(Double(n)))
+        return sqN * sqN == n
+    }
+    
+    func numSquares(_ n: Int) -> Int {
+        guard n > 0 else {
+            return 0
+        }
+        for i in 1...n {
+            if isPerfectSquare(n: i) {
+                perfectSquare.append(i)
+                cache[i] = 1
+            }
+        }
+        for i in 1...n {
+            if findSum(count: i, num: n) {
+                return i
+            }
+        }
+        return n
+    }
+}
+```