Merge pull request #91 from remy727/0486-predict-the-winner

486. Predict the Winner

Author: remy727

README.md

@@ -193,6 +193,7 @@
 | 445 | Add Two Numbers II | [Ruby](./algorithms/ruby/0445-add-two-numbers-ii.rb) | Medium |
 | 450 | Delete Node in a BST | [Ruby](./algorithms/ruby/0450-delete-node-in-a-bst.rb) | Medium |
 | 452 | Minimum Number of Arrows to Burst Balloons | [Ruby](./algorithms/ruby/0452-minimum-number-of-arrows-to-burst-balloons.rb) | Medium |
+| 486 | Predict the Winner | [Ruby](./algorithms/ruby/0486-predict-the-winner.rb) | Medium |
 | 502 | IPO | [Ruby](./algorithms/ruby/0502-ipo.rb) [Python3](./algorithms/python3/0502-ipo.py) | Hard |
 | 509 | Fibonacci Number | [Ruby](./algorithms/ruby/0509-fibonacci-number.rb) | Easy |
 | 516 | Longest Palindromic Subsequence | [Ruby](./algorithms/ruby/0516-longest-palindromic-subsequence.rb) | Medium |

algorithms/ruby/0486-predict-the-winner.rb

@@ -0,0 +1,61 @@
+# frozen_string_literal: true
+
+# 486. Predict the Winner
+# Medium
+# https://leetcode.com/problems/predict-the-winner
+
+=begin
+You are given an integer array nums. Two players are playing a game with this array: player 1 and player 2.
+Player 1 and player 2 take turns, with player 1 starting first. Both players start the game with a score of 0. At each turn, the player takes one of the numbers from either end of the array (i.e., nums[0] or nums[nums.length - 1]) which reduces the size of the array by 1. The player adds the chosen number to their score. The game ends when there are no more elements in the array.
+Return true if Player 1 can win the game. If the scores of both players are equal, then player 1 is still the winner, and you should also return true. You may assume that both players are playing optimally.
+
+Example 1:
+Input: nums = [1,5,2]
+Output: false
+Explanation: Initially, player 1 can choose between 1 and 2.
+If he chooses 2 (or 1), then player 2 can choose from 1 (or 2) and 5. If player 2 chooses 5, then player 1 will be left with 1 (or 2).
+So, final score of player 1 is 1 + 2 = 3, and player 2 is 5.
+Hence, player 1 will never be the winner and you need to return false.
+
+Example 2:
+Input: nums = [1,5,233,7]
+Output: true
+Explanation: Player 1 first chooses 1. Then player 2 has to choose between 5 and 7. No matter which number player 2 choose, player 1 can choose 233.
+Finally, player 1 has more score (234) than player 2 (12), so you need to return True representing player1 can win.
+
+Constraints:
+* 1 <= nums.length <= 20
+* 0 <= nums[i] <= 107
+=end
+
+# @param {Integer[]} nums
+# @return {Boolean}
+def predict_the_winner(nums)
+  n = nums.length
+  dp = Array.new(n) { Array.new(n, 0) }
+  nums.inject(:+)
+
+  n.downto(0).each do |i|
+    (i...n).each do |j|
+      if i == j
+        dp[i][j] = nums[i]
+      else
+        dp[i][j] = [nums[i] - dp[i + 1][j], nums[j] - dp[i][j - 1]].max
+      end
+    end
+  end
+
+  dp[0][n - 1] >= 0
+end
+
+# **************** #
+#       TEST       #
+# **************** #
+
+require "test/unit"
+class Test_predict_the_winner < Test::Unit::TestCase
+  def test_
+    assert_equal false, predict_the_winner([1, 5, 2])
+    assert_equal true, predict_the_winner([1, 5, 233, 7])
+  end
+end