title: Why is this called backtracking?
body: <p>I have read in Wikipedia and have also Googled it, </p>
  
  <p>but I cannot figure out what "Backtracking Algorithm" means.</p>
  
  <p>I saw this solution from "Cracking the Code Interviews"</p>
  
  <p>and wonder why is this a backtracking algorithm?</p>
  
  <p><img src="https://i.stack.imgur.com/fDMDs.png" alt="enter image description here"></p>