title: java replace substring in string specific index
body: <p>How would I replace a string <code>10100</code> with <code>10010</code> using the algorithm "replace the last substring 10 with 01."
  I tried </p>
  
  <pre><code>s=s.replace(s.substring(a,a+2), "01");
  </code></pre>
  
  <p>but this returns <code>01010</code>, replacing both the first and the second substring of <code>"10"</code>.
  "a" represents s.lastindexOf("10");</p>