Merge branch 'master' of https://github.com/anebz/ctci

05. Bit manipulation/5.5. Debugger.md

@@ -0,0 +1,57 @@
+# 5.5. Debugger
+
+> Explain what the following code does: (( n & (n-1)) == 0)
+
+```c++
+  00000010   (n)
+& 00000001   (n - 1)
+----------
+  00000000
+```
+
+n should be odd. And if it's bigger than 2, it'll have some 1s higher up. Which won't be cleared when doing n-1. so n must be 2.
+
+## Hints
+
+> Start with a brute force solution. Can you try all possibilities?
+
+n=2, but there must be something more.
+
+> What does it mean if A & B == 0?
+
+That A and B don't have any same bits in any position.
+
+> If A & B == 0, then it means that A and B never have a 1 at the same spot. Apply this to the equation in the problem.
+
+A and B are separated by 1 bit in this case, A = B + 1. It could be all 0s and 10 in the end (n=2), or 100000. So that n = pow(2,x). A power of 2.
+
+```c++
+  10000000   (n)
+& 01111111   (n - 1)
+----------
+  00000000
+```
+
+> If ( n & ( n-1)) == 0, then this means that n and n - 1 never have a 1 in the same spot. Why would that happen?
+
+When n is a power of 2.
+
+> What is the relationship between how n looks and how n - 1 looks? Walk through a binary subtraction.
+
+Yes.
+
+> When you do a binary subtraction, you flip the rightmost 0s to a 1, stopping when you get to a 1 (which is also flipped). Everything (all the 1 s and Os) on the left will stay put.
+
+Same answer.
+
+> Picture n and n - 1. To subtract 1 from n, you flipped the rightmost 1 to a 0 and all the 0s on its right to 1s. If n & n -1 == 0, then there are no 1 s to the left of the first 1. What does that mean about n?
+
+I'm getting lost.
+
+> We know that n must have only one 1 ifn & ( n -1) == 0. What sorts ofnumbers have only one 1?
+
+Powers of 2! Or the binary base number.
+
+## Solution
+
+Correct! n = a power of 2, or n=0.

05. Bit manipulation/5.6. Conversion.md

@@ -0,0 +1,32 @@
+# 5.6. Conversion
+
+> Write a function to determine the number of bits you would need to flip to convert integer A to integer B
+
+Input: 29 (or: 11101), 15 (or: 01111)
+Output: 2
+
+First need to convert to array of bits, right? Levenstein distance between them. Or just iterate the bits by doing num >> 1, get rightmost bit of both nums and do XOR. Sum up all numbers.
+
+## Hints
+
+> How would you figure out how many bits are different between two numbers?
+
+Levenstein distance between the strings, but we're working with numbers. Iterate all bits and add up the differences.
+
+> Think about what an XOR indicates. If you do a XOR b, where does the result have 1s? Where does it have Os?
+
+XOR has result 0 if bits are different, 1 if bits are same.
+
+## Solution
+
+a ^ b makes XOR between a and b, then just iterate a c and increase count if the value at that point is 1. Then shift c to right, keep deleting the right most bit.
+
+```c++
+int bitSwapRequired(int a, int b) {
+    int count = 0;
+    for (int c = a ^ b; c != 0; c = c >> 1) {
+        count += c & 1;
+    }
+    return count;
+}
+```

Missing CS semester/README.md

@@ -186,7 +186,7 @@ To enter **Command line mode**, type `:`.
 * o / O insert line below / above
 * `A` append to line (in the end of line)
 * `d{motion}` delete {motion}
-  * e.g. dw is delete word, d$ is delete to end of line, d0 is* delete to beginning of line, d$ delete until end of line
+  * e.g. dw is delete word, d$ is delete to end of line, d0 is delete to beginning of line, d$ delete until end of line
 * `dd` delete whole line. 2dd, delete this and next line
 * `rx` to replace the character at the cursor by x. `ra`, deletes current character and writes a.
 * c{motion} change {motion}
@@ -199,7 +199,7 @@ To enter **Command line mode**, type `:`.
 * visual mode + manipulation
   * select text, d to delete it or c to change it
 * `u` to undo, `U` to undo whole line, `<Ctr> + R` to redo
-* y to copy / “yank” (some other commands like d also copy)
+* `y` to copy / “yank” (some other commands like d also copy)
 * `p` to paste under the cursor
 * Lots more to learn: e.g. ~ flips the case of a character
 

README.md

@@ -53,6 +53,9 @@ If you can't afford to buy the book, you can find a free pdf [here](http://ahmed
 
 * Insert bit
 * Binary to string
+* Flip bit to create longest sequence of 1s
+* (( n & (n-1)) == 0). n=?
+* How many different bits do two numbers have. 11001 vs. 11010 = 2. Levenstein distance
 
 ## Chapter 7 Object-oriented design