{"id":10835,"date":"2018-12-31T20:57:43","date_gmt":"2018-12-31T20:57:43","guid":{"rendered":"http:\/\/www.softwareeverydayblog.com\/?p=10835"},"modified":"2018-12-31T20:58:53","modified_gmt":"2018-12-31T20:58:53","slug":"leetcode-random-pick-index","status":"publish","type":"post","link":"https:\/\/www.softwareeverydayblog.com\/?p=10835","title":{"rendered":"[Leetcode] Random Pick Index"},"content":{"rendered":"<p>Given an array of integers with possible duplicates, randomly output the index of a given target number. You can assume that the given target number must exist in the array.<\/p>\n<p><strong>Note<\/strong>:<br \/>\nThe array size can be very large. Solution that uses too much extra space will not pass the judge.<\/p>\n<p><strong>Example<\/strong>:<\/p>\n<pre lang=\"text\">\r\nint[] nums = new int[] {1,2,3,3,3};\r\nSolution solution = new Solution(nums);\r\n\r\n\/\/ pick(3) should return either index 2, 3, or 4 randomly. Each index should have equal probability of returning.\r\nsolution.pick(3);\r\n\r\n\/\/ pick(1) should return 0. Since in the array only nums[0] is equal to 1.\r\nsolution.pick(1);\r\n<\/pre>\n<p>[<a href=\"https:\/\/leetcode.com\/problems\/random-pick-index\/description\/\" rel=\"noopener\" target=\"_blank\">Problem Link<\/a>]<\/p>\n<ul>\n<li>Create a Map for every target -> range[]. Now, given a target we can query its range in O(1) time. Note, that length of this range is simply range[1]-range[0]+1.<\/li>\n<li>Given a range (for any target), generate a random_number between 0 to length_of_range (Java&#8217;s Random class allows us to generate a random number between 0 (inclusive) to n (exclusive)). We then simple return i + random_number.<\/li>\n<\/ul>\n<pre lang=\"java\">\r\nclass Solution {\r\n    \r\n    Map<Integer, int[]> rangeMap;\r\n    Random r;\r\n    \r\n    public Solution(int[] nums) {\r\n        r = new Random();\r\n        rangeMap = new HashMap<Integer, int[]>();\r\n        int i = 0;\r\n        while ( i < nums.length ) {\r\n            int num = nums[i];\r\n            int j = i;\r\n            while ( j < nums.length &#038;&#038; nums[j] == num )\r\n                j++;\r\n            int range[] = { i, j-1 };\r\n            rangeMap.put(num, range);\r\n            i = j;\r\n        }        \r\n    }\r\n    \r\n    public int pick(int target) {\r\n        int range[] = rangeMap.get(target);\r\n        return range[0] + r.nextInt(range[1]-range[0]+1);\r\n    }\r\n}\r\n\r\n\/**\r\n * Your Solution object will be instantiated and called as such:\r\n * Solution obj = new Solution(nums);\r\n * int param_1 = obj.pick(target);\r\n *\/\r\n<\/pre>\n","protected":false},"excerpt":{"rendered":"<p>Given an array of integers with possible duplicates, randomly output the index of a given target number. You can assume that the given target number must exist in the array. Note: The array size can be very large. Solution that uses too much extra space will not pass the judge. Example: int[] nums = new [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[],"class_list":["post-10835","post","type-post","status-publish","format-standard","hentry","category-uncategorized"],"_links":{"self":[{"href":"https:\/\/www.softwareeverydayblog.com\/index.php?rest_route=\/wp\/v2\/posts\/10835","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.softwareeverydayblog.com\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.softwareeverydayblog.com\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.softwareeverydayblog.com\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.softwareeverydayblog.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=10835"}],"version-history":[{"count":3,"href":"https:\/\/www.softwareeverydayblog.com\/index.php?rest_route=\/wp\/v2\/posts\/10835\/revisions"}],"predecessor-version":[{"id":10837,"href":"https:\/\/www.softwareeverydayblog.com\/index.php?rest_route=\/wp\/v2\/posts\/10835\/revisions\/10837"}],"wp:attachment":[{"href":"https:\/\/www.softwareeverydayblog.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=10835"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.softwareeverydayblog.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=10835"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.softwareeverydayblog.com\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=10835"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}