Largest Number After Mutating Substring LeetCode solution in Java

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class Solution {
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  public String maximumNumber(String num, int[] change) {
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    int i = 0, n = num.length(), startIndex = -1, substringLength = 0;
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    // traverse through each digit in the input string
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    while (i < n) {
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      int digit = num.charAt(i) - 48;
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      // when we encounter a digit which has greater change
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      if (change[digit] > digit) {
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        startIndex = i;
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        // keep on replacing subsequent characters with with the change if they also have greater
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        // change
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        while (i < n) {
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          digit = num.charAt(i) - 48;
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          if (change[digit] < digit) {
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            break;
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          }
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          i += 1;
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        }
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        substringLength = i - startIndex;
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        break;
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      }
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      i += 1;
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    }
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    // Note: Using String Builder to ensure linear time complexity as java strings are immutable
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    StringBuilder result = new StringBuilder("");
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    for (int j = 0; j < n; j++) {
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      int digit = num.charAt(j) - 48;
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      if (j >= startIndex && j < startIndex + substringLength) {
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        result.append(change[digit]);
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      } else {
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        result.append(digit);
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      }
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    }
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    return result.toString();
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  }
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}