Map Sum Pairs LeetCode solution in Java

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class MapSum {
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  private Map<String, Integer> map;
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  private Map<String, Integer> words;
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  public MapSum() {
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    this.map = new HashMap<>();
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    this.words = new HashMap<>();
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  }
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  public void insert(String key, int val) {
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    Integer lookup = this.words.getOrDefault(key, null);
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    int diff;
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    if (lookup == null) diff = val;
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    else diff = val - lookup;
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    int k = key.length();
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    StringBuilder sb = new StringBuilder();
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    for (int i = 0; i < k; i++) {
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      sb.append(key.charAt(i));
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      String prefix = sb.toString();
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      map.put(prefix, map.getOrDefault(prefix, 0) + diff);
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    }
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    this.words.put(key, val);
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  }
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  public int sum(String prefix) {
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    return this.map.getOrDefault(prefix, 0);
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  }
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}
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/**
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 * Your MapSum object will be instantiated and called as such: MapSum obj = new MapSum();
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 * obj.insert(key,val); int param_2 = obj.sum(prefix);
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 */