Adding Spaces to a String LeetCode solution in Java

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class Solution {
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  public String addSpaces(String s, int[] spaces) {
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    StringBuilder sb = new StringBuilder();
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    int k = 0;
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    for (int i = 0; i < spaces.length; i++) {
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      sb.append(s.substring(k, spaces[i]));
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      k = spaces[i];
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      sb.append(" ");
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    }
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    sb.append(s.substring(k, s.length()));
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    return sb.toString();
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  }
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}