add solution of problem 69: sqrt(x)

Author: ppulse

Sqrt(x)69/README.md

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+Implement `int sqrt(int x)`.
+
+Compute and return the square root of *x*.

Sqrt(x)69/Solution.java

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+public class Solution {
+    public int mySqrt(int x) {
+        if (x < 0)
+            return -1;
+        else if (x == 0)
+            return 0;
+        
+        long left = 1;
+        long right = x;
+        
+        while (left < right) {
+            long mid = (left + right) >> 1;
+            long val = mid * mid;
+            
+            if (val > x) {
+                right = mid;
+            } else if (val < x && mid > left) {
+                left = mid;
+            } else {
+                return (int)mid;
+            }
+        }
+        
+        return (int)left;
+    }
+}