add

Author: houbb

src/main/java/com//houbb/leetcode/f500t600/T560_subarraySum_V1_prefixSumHashMap.java

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+package com..houbb.leetcode.f500t600;
+
+import java.util.HashMap;
+import java.util.Map;
+
+public class T560_subarraySum_V1_prefixSumHashMap {
+
+
+public int subarraySum(int[] nums, int k) {
+int n = nums.length;
+
+// 前缀和数组
+int[] prefixSum = new int[n];
+prefixSum[0] = nums[0];
+for (int i = 1; i < n; i++) {
+prefixSum[i] = prefixSum[i-1] + nums[i];
+}
+
+// 哈希表记录前缀和第一次出现的位置
+Map<Integer, Integer> map = new HashMap<>();
+// 从零开始的计算 下标为0的元素，前缀和为0，数量为1
+map.put(0, 1);
+int count = 0;
+
+// 遍历前缀和数组
+for (int i = 0; i < n; i++) {
+// 查找前缀和减去 k 的位置，则说明满足等于 k
+int sum = prefixSum[i];
+if (map.containsKey(sum - k)) {
+count += map.get(sum - k);
+}
+
+map.put(sum, map.getOrDefault(sum, 0) + 1);
+}
+
+return count;
+}
+
+}