Add math solution for leetcode 279.

Author: yanglr

cpp-leetcode/leetcode279-perfect-squares_math-method.cpp

@@ -0,0 +1,40 @@
+#include<iostream>
+#include<cmath>
+using namespace std;
+
+/* 解题思路见题解 <https://leetcode-cn.com/problems/perfect-squares/solution/shu-xue-fa-bu-shi-yong-dong-tai-gui-hua-ddm33/> */
+class Solution {
+public:
+    int numSquares(int n) {
+        while (n % 4 == 0)
+            n /= 4;
+
+        if (n % 8 == 7)
+            return 4;   // (path 1)
+
+        for (int i = 0; i * i <= n; i++)
+        {
+            int r = (int)sqrt(n - i * i);
+            if (i * i + r * r == n)
+            {
+                if (i == 0 || r == 0)
+                    return 1;           // (path 2)
+
+                return 2;   // (path 3)
+            }
+        }
+
+        return 3;   // (path 4)
+    }
+};
+
+// Test
+int main()
+{
+    Solution sol;
+    int k = 12;
+    int res = sol.numSquares(k);
+    cout << res << endl;
+
+    return 0;
+}