Adding math algorithm to compute power and its tests (trekhleb#172)

* Adding math algorithm to compute power and its tests

* adding more test cases, updating compute power js

* Updating ReadMe for power computation algorithm

Author: HatimLokhandwala

src/algorithms/math/compute-power/README.md

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+# Power(a,b)
+
+This computes power of (a,b)
+eg: power(2,3) = 8
+power(10,0) = 1
+
+The algorithm uses divide and conquer approach to compute power.
+Currently the algorithm work for two positive integers X and Y
+Lets say there are two numbers X and Y.
+At each step of the algorithm:
+  1. if Y is even
+      then power(X, Y/2) * power(X, Y/2) is computed
+  2. if Y is odd  
+      then X * power(X, Y/2) * power(X, Y/2) is computed
+
+At each step since power(X,Y/2) is called twice, this is optimised by saving the result of power(X, Y/2) in a variable (lets say res).
+And then res is multiplied by self.
+
+Illustration through example
+power (2,5)
+  - 2 * power(2,2) * power(2,2)
+  power(2,2)
+    - power(2,1) * power(2,1)
+    power(2,1)
+      - return 2
+
+Going up the tree once the end values are computed
+    power(2,1) = 2
+  power(2,2) = power(2,1) * power(2,1) = 2 * 2 = 4  
+power(2,5) = 2 * power(2,2) * power(2,2) = 2 * 4 * 4 = 32
+
+
+Complexity relation: T(n) = T(n/2) + 1
+
+Time complexity of the algorithm: O(logn)

src/algorithms/math/compute-power/__test__/computePower.test.js

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+import computePower from '../power';
+
+describe('computePower', () => {
+  it('should compute Power', () => {
+    expect(computePower(1, 1)).toBe(1);
+    expect(computePower(2, 0)).toBe(1);
+    expect(computePower(3, 4)).toBe(81);
+    expect(computePower(190, 2)).toBe(36100);
+    expect(computePower(16, 16)).toBe(18446744073709552000);
+    expect(computePower(100, 9)).toBe(1000000000000000000);
+    expect(computePower(9, 16)).toBe(1853020188851841);
+    expect(computePower(11, 5)).toBe(161051);
+    expect(computePower(13, 11)).toBe(1792160394037);
+    expect(computePower(7, 21)).toBe(558545864083284000);
+  });
+});

src/algorithms/math/compute-power/power.js

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+/**
+ * @param {number1} number
+ * @param {number2} number
+ * @return {number1^number2}
+ */
+
+// recursive implementation to compute power
+export default function computePower(number1, number2) {
+  let val = 0;
+  let res = 0;
+  if (number2 === 0) { // if number2 is 0
+    val = 1;
+  } else if (number2 === 1) { // if number2 is 1 return number 1 as it is
+    val = number1;
+  } else if (number2 % 2 === 0) { // if number2 is even
+    res = computePower(number1, number2 / 2);
+    val = res * res;
+  } else { // if number2 is odd
+    res = computePower(number1, Math.floor(number2 / 2));
+    val = res * res * number1;
+  }
+  return val;
+}