Add README for day2

day2/README.md

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+# Day 2: 1202 Program Alarm
+
+## Problem
+
+### Part One
+
+On the way to your gravity assist around the Moon, your ship computer beeps
+angrily about a "1202 program alarm". On the radio, an Elf is already
+explaining how to handle the situation: "Don't worry, that's perfectly norma--"
+The ship computer bursts into flames.
+
+You notify the Elves that the computer's magic smoke seems to have escaped.
+"That computer ran Intcode programs like the gravity assist program it was
+working on; surely there are enough spare parts up there to build a new Intcode
+computer!"
+
+An Intcode program is a list of integers separated by commas (like 1,0,0,3,99).
+To run one, start by looking at the first integer (called position 0). Here,
+you will find an opcode - either 1, 2, or 99. The opcode indicates what to do;
+for example, 99 means that the program is finished and should immediately halt.
+Encountering an unknown opcode means something went wrong.
+
+Opcode 1 adds together numbers read from two positions and stores the result in
+a third position. The three integers immediately after the opcode tell you
+these three positions - the first two indicate the positions from which you
+should read the input values, and the third indicates the position at which the
+output should be stored.
+
+For example, if your Intcode computer encounters 1,10,20,30, it should read the
+values at positions 10 and 20, add those values, and then overwrite the value
+at position 30 with their sum.
+
+Opcode 2 works exactly like opcode 1, except it multiplies the two inputs
+instead of adding them. Again, the three integers after the opcode indicate
+where the inputs and outputs are, not their values.
+
+Once you're done processing an opcode, move to the next one by stepping forward
+4 positions.
+
+For example, suppose you have the following program:
+
+1,9,10,3,2,3,11,0,99,30,40,50
+
+For the purposes of illustration, here is the same program split into multiple
+lines:
+
+1,9,10,3,
+2,3,11,0,
+99,
+30,40,50
+
+The first four integers, 1,9,10,3, are at positions 0, 1, 2, and 3. Together,
+they represent the first opcode (1, addition), the positions of the two inputs
+(9 and 10), and the position of the output (3). To handle this opcode, you
+first need to get the values at the input positions: position 9 contains 30,
+and position 10 contains 40. Add these numbers together to get 70. Then, store
+this value at the output position; here, the output position (3) is at position
+3, so it overwrites itself. Afterward, the program looks like this:
+
+1,9,10,70,
+2,3,11,0,
+99,
+30,40,50
+
+Step forward 4 positions to reach the next opcode, 2. This opcode works just
+like the previous, but it multiplies instead of adding. The inputs are at
+positions 3 and 11; these positions contain 70 and 50 respectively. Multiplying
+these produces 3500; this is stored at position 0:
+
+3500,9,10,70,
+2,3,11,0,
+99,
+30,40,50
+
+Stepping forward 4 more positions arrives at opcode 99, halting the program.
+
+Here are the initial and final states of a few more small programs:
+
+* 1,0,0,0,99 becomes 2,0,0,0,99 (1 + 1 = 2).
+* 2,3,0,3,99 becomes 2,3,0,6,99 (3 * 2 = 6).
+* 2,4,4,5,99,0 becomes 2,4,4,5,99,9801 (99 * 99 = 9801).
+* 1,1,1,4,99,5,6,0,99 becomes 30,1,1,4,2,5,6,0,99.
+
+Once you have a working computer, the first step is to restore the gravity
+assist program (your puzzle input) to the "1202 program alarm" state it had
+just before the last computer caught fire. To do this, before running the
+program, replace position 1 with the value 12 and replace position 2 with the
+value 2. What value is left at position 0 after the program halts?