{"id":2559,"date":"2018-05-07T21:03:54","date_gmt":"2018-05-07T13:03:54","guid":{"rendered":"https:\/\/boweihe.me\/?p=2559"},"modified":"2018-05-07T21:03:54","modified_gmt":"2018-05-07T13:03:54","slug":"leetcode-560-subarray-sum-equals-k","status":"publish","type":"post","link":"https:\/\/dayandcarrot.space\/?p=2559","title":{"rendered":"LeetCode 560. Subarray Sum Equals K"},"content":{"rendered":"<p><a href=\"https:\/\/leetcode.com\/problems\/subarray-sum-equals-k\/description\/\" target=\"_blank\" rel=\"noopener noreferrer\">https:\/\/leetcode.com\/problems\/subarray-sum-equals-k\/description\/<\/a><br \/>\nO(N^2)\u7684\u65b9\u6cd5\u60f3\u5fc5\u6240\u6709\u4eba\u90fd\u4f1a\uff0c\u8fd9\u91cc\u6709\u4e2a\u7528\u5230<a href=\"https:\/\/en.wikipedia.org\/wiki\/Prefix_sum\" target=\"_blank\" rel=\"noopener noreferrer\">prefix sum<\/a>\u7684O(N)\u65b9\u6cd5\u3002<br \/>\n\u8bd5\u60f3\u4e00\u4e0b\uff0c\u5982\u679c\u5f53\u524d\u68c0\u67e5\u5230end\u4f4d\u7f6e\u4e14\u76ee\u524d\u6240\u6709\u9879\u7684\u548c\u4e3asum\uff0c\u5982\u679c\u6709\u90a3\u4e48\u4e00\u4e2a(start, end)\u5b50\u5e8f\u5217\u7684\u548c\u4e3ak\uff0c\u90a3\u4e48\u80af\u5b9a\u6709\u4e00\u4e2a(0, start-1)\u7684\u5e8f\u5217\u5b83\u7684\u548c\u662fsum &#8211; k.<\/p>\n<pre class=\"lang:default decode:true\">[a, b, c] \\___k__\/\n[a, b, c, d, e, f]\n\/sum - k\\<\/pre>\n<pre class=\"lang:c++ decode:true \">class Solution {\npublic:\n    int subarraySum(vector&lt;int&gt;&amp; nums, int k) {\n        unordered_map&lt;int, int&gt; cum_sums;   \/\/ &lt;cum_sum, count&gt;\n        cum_sums[0] = 1;\n        int sum = 0;\n        int count = 0;\n        for (int i = 0; i &lt; nums.size(); ++i)\n        {\n            sum += nums[i];\n            if (cum_sums.count(sum - k) &gt; 0)\n                count += cum_sums[sum - k];\n            cum_sums[sum]++;\n        }\n        return count;\n    }\n};<\/pre>\n<p>&nbsp;<\/p>\n","protected":false},"excerpt":{"rendered":"<p>https:\/\/leetcode.com\/problems\/subarray-sum-equals-k\/description\/ O(N^2)\u7684\u65b9\u6cd5\u60f3\u5fc5\u6240\u6709\u4eba\u90fd\u4f1a\uff0c\u8fd9\u91cc\u6709\u4e2a\u7528\u5230prefix sum\u7684O(N)\u65b9\u6cd5\u3002 \u8bd5\u60f3\u4e00\u4e0b\uff0c\u5982\u679c\u5f53\u524d\u68c0\u67e5\u5230end\u4f4d\u7f6e\u4e14\u76ee\u524d\u6240\u6709\u9879\u7684\u548c\u4e3asum\uff0c\u5982\u679c\u6709\u90a3\u4e48\u4e00\u4e2a(start, end)\u5b50\u5e8f\u5217\u7684\u548c\u4e3ak\uff0c\u90a3\u4e48\u80af\u5b9a\u6709\u4e00\u4e2a(0, start-1)\u7684\u5e8f\u5217\u5b83\u7684\u548c\u662fsum &#8211; k. [a, b, c] \\___k__\/ [a, b, c, d, e, f] \/sum &#8211; k\\ class Solution { public: int subarraySum(vector&lt;int&gt;&amp; nums, int k) { unordered_map&lt;int, int&gt; cum_sums; \/\/ &lt;cum_sum, count&gt; cum_sums[0] = 1; int sum = 0; int count = 0; for (int i = 0; i &lt; [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[4],"tags":[66],"class_list":["post-2559","post","type-post","status-publish","format-standard","hentry","category-study","tag-leetcode-oj"],"_links":{"self":[{"href":"https:\/\/dayandcarrot.space\/index.php?rest_route=\/wp\/v2\/posts\/2559","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/dayandcarrot.space\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/dayandcarrot.space\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/dayandcarrot.space\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/dayandcarrot.space\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=2559"}],"version-history":[{"count":0,"href":"https:\/\/dayandcarrot.space\/index.php?rest_route=\/wp\/v2\/posts\/2559\/revisions"}],"wp:attachment":[{"href":"https:\/\/dayandcarrot.space\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=2559"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/dayandcarrot.space\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=2559"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/dayandcarrot.space\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=2559"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}