Number of Pairs of Strings With Concatenation Equal to Target LeetCode solution in Java

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class Solution {
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  public int numOfPairs(String[] nums, String target) {
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    HashMap<String, Integer> map = new HashMap<>();
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    for (int i = 0; i < nums.length; i++) {
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      map.put(nums[i], map.getOrDefault(nums[i], 0) + 1);
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    }
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    int ans = 0, n = target.length();
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    String a = "", b = "";
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    for (int i = 1; i < n; i++) {
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      a = target.substring(0, i);
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      b = target.substring(i, n);
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      if (map.containsKey(a) && map.containsKey(b)) {
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        if (a.equals(b)) ans += (map.get(a) * (map.get(a) - 1));
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        else ans += (map.get(a) * map.get(b));
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      }
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    }
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    return ans;
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  }
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}