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HTML 

              
                <div id="grid"></div>
<div id="numbers">
  <div id="row1" class="row">
    <div id="one" class="number"><b>1</b></div>
    <div id="two" class="number"><b>2</b></div>
    <div id="three" class="number"><b>3</b></div>
    <div id="four" class="number"><b>4</b></div>
    <div id="five" class="number"><b>5</b></div>
  </div>
  <div id="row2" class="row">
    <div id="six" class="number"><b>6</b></div>
    <div id="seven" class="number"><b>7</b></div>
    <div id="eight" class="number"><b>8</b></div>
    <div id="nine" class="number"><b>9</b></div>
  </div>
</div>
<h2>Sudoku</h2>
<div id="footer">
  <p>coded by
    <a href="https://codepen.io/haben/#" target="_blank">haben</a>
  </p>
</div>
!

CSS 

              
                body {
  background-color: #405952;
  color: #9c9b7a;
  text-align: center;
  font-family: "Arvo", serif;
}

h4 {
  padding: 5px;
}

a {
  color: #f54f29;  
}

a:hover {
  text-decoration: none;
  color: #ff974f;
}

.row {
  margin: auto;
}

.square {
  height: 30px;
  width: 30px;
  text-align: center;
  border: 1px solid #9c9b7a;
  color: #405952;
  font-size: 20px;
  padding: auto;
  display: inline-block;
}

.hint {
  height: 8px;
  width: 8px;
  text-align: center;
  color: #9c9b7a;
  font-size: 8px;
}

.number {
  height: 40px;
  width: 40px;
  text-align: center;
  border: 1px solid #9c9b7a;
  color: #405952;
  font-size: 25px;
  margin: 5px;
  border: 2px solid #405952;
  border-radius: 10px;
  background-color: #ffd393;
  display: inline-block;
}

#grid {
  height: 320px;
  width: 320px;
  background-color: #ffd393;
  margin: 10px auto;
  padding: 25px;
}

#numbers {
  height: 150px;
  width: 320px;
  margin: 10px auto;
}

#row1 {
  margin: 0px 35px;
}

#row2 {
  margin: 0px 60px;
}
!

JS 

              
                var grid;
var digits;
var hints;
var order = [];
var puzzle = [];
var active = "one";
var mode = "pick";

function emptyGrid() {
  grid = [];
  for (var i = 0; i < 81; i++) {
    grid[i] = 0;
  }
}

function getHints() {
  hints = [];
  for (var i = 0; i < 81; i++) {
    hints[i] = new Array();
    for (var j = 0; j < 9; j++) {
      hints[i][j] = 0;
    }
  }  
}

function getDigits() {
  var ordered = [];
  digits = [];
  
  for (var i = 1; i <= 9; i++) {
    ordered.push(i);
  }
  
  while (ordered.length > 0) {
    var i = Math.floor(Math.random() * ordered.length);
    digits.push(ordered[i]);
    ordered.splice(i, 1);
  }
}

function isFull() {
  for (var i = 0; i < 81; i++) {
    if (grid[i] === 0) {
      return false;
    }
  }
  return true;
}

function generateGrid() {
  fillFirstBox();
  fillSecondBox();
  finishBox(2);
  fillFirstColumn();
  fillLeast();
}

function fill(spot, n) {
  grid[spot] = n;
  //updateHints(spot);
  order.push(spot);
}

function fillFirstBox() {
  getDigits();
  for (var row = 0; row < 3; row++) {
    for (var col = 0; col < 3; col++) {
      var n = digits[digits.length-1];
      fill(row * 9 + col, n);
      digits.pop();
    }
  }
}

function fillSecondBox() {
  getDigits();
  digits = digits.filter(function(el){return isSafe(3, el);}); 
  for (var i = 0; i < 3; i++) {
    fill(i + 3, digits.pop());
  }

  fillThreeRow(12, 21);
  fillThreeRow(21, 12);
  
  finishBox(1);
}

function fillThreeRow(allow, block) {
  getDigits();
  var j = digits.length - 1;
  while (j >= 0) {
    if (!isSafe(allow, digits[j]) || isSafe(block, digits[j])) {
      digits.splice(j, 1);
    }
    j--;
  }
  for (var i = 0; i < digits.length; i++) {
    fill(allow + i, digits[i]);
  }
}

function fillFirstColumn() {
  getDigits();
  digits = digits.filter(function(el){return isSafe(27, el);});
  for (var i = 0; i < digits.length; i++) {
    fill(27 + i * 9, digits[i]);
  }
}

function fillLeftColumns() {
  fillThreeColumn(28, 55);
  fillThreeColumn(55, 28);
  fillThreeColumn(29, 56);
  fillThreeColumn(56, 29);
  finishBox(3);
  finishBox(6);
}

function fillThreeColumn(allow, block) {
  getDigits();
  var j = digits.length - 1;
  while (j >= 0) {
    if (!isSafe(allow, digits[j]) || isSafe(block, digits[j])) {
      digits.splice(j, 1);
    }
    j--;
  }
  for (var i = 0; i < digits.length; i++) {
    fill(allow + i * 9, digits[i]);
  }
}

function finishBox(box) {
  var begin = Math.floor(box / 3) * 27 + Math.floor(box % 3) * 3;
  for (var row = 0; row < 3; row++) {
    for (var col = 0; col < 3; col++) {
      var spot = begin + row * 9 + col;
      if (grid[spot] === 0) {
        getDigits();
        var i = digits.length - 1;
        while (i >= 0) {
          if (!isSafe(spot, digits[i])) {
            digits.splice(i, 1);
          }
          i--;
        }
        fill(spot, digits.pop());
      }
    }
  }
}

function fillLeast() {
  for (var h = 0; h < 48; h++) {
    var least = 10;
    var spot = 0;
    for (var i = 0; i < 81; i++) {
      if (grid[i] === 0) {
        var candidates = countCandidates(i);
        if (candidates < least) {
          least = candidates;
          spot = i;
        }
      }
    }
    getDigits();
    digits = digits.filter(function(el){return isSafe(spot, el);});
    if (digits.length > 0) {
      fill(spot, digits.pop());
    } else {  // here is where I would backtrack
    }
  }
}

function countCandidates(spot) {
  var candidates = 0;
  for (var i = 0; i < 9; i++) {
    if (hints[spot][i] !== 0) {
      candidates++;
    }
  }
  return candidates;
}

function isSafe(i, digit) {
  return checkRow(i, digit) && checkColumn(i, digit) && checkBox(i, digit);
}

function checkRow(i, digit) {
  var row = Math.floor(i / 9);
  for (var col = 0; col < 9; col++) {
    if (grid[row * 9 + col] === digit) {
      return false;
    }
  }
  return true;
}

function checkColumn(i, digit) {
  var col = i % 9;

  for (var row = 0; row < 9; row++) {
    if (grid[row * 9 + col] === digit) {
      return false;
    }
  }
  return true;
}
  
function checkBox(i, digit) {
  var begin = Math.floor(i / 27) * 27 + Math.floor((i % 9) / 3) * 3;
  for (var row = 0; row < 3; row++) {
    for (var col = 0; col < 3; col++) {
      if (grid[begin + (row * 9) + col] === digit) {
        return false;
      }
    }
  }
  return true;  
}

function getBox(n) {
  return Math.floor(n / 27) * 3 + Math.floor(n % 9 / 3);
}

function printGrid() {
  var str = "";
  for (var row = 0; row < 9; row++) {
    str += '<div class="row">';
    for (var col = 0; col < 9; col++) {
      str += printSquare(row * 9 + col);
    }
    str += "</div>";
  }
  $("#grid").html(str);
  colorBorders();
}

function printSquare(spot) {
  var str = "<div class=\"square\" ";
  str += "id=\"" + spot.toString() + "\"";
  if (puzzle[spot] === 0) {
    str += "style=\"color:#f54f29\"";
  }
  str += ">";
  if (grid[spot] === 0) {
    str += printHints(spot); 
  } else {
    str += "<b>" + grid[spot].toString() + "</b>";
  }
  str += "</div>";

  return str;
}

function printHints(spot) {
  var str = "";
  
  for (var row = 0; row < 3; row++) {
    str += "<div class=\"row\">";
    for (var col = 0; col < 3; col++) {
      var n = hints[spot][row * 3 + col];
      str += "<div class=\"hint\">";
      if (n !== 0) {
        str += hints[spot][row * 3 + col].toString();
      }
      str += "</div>";
    }
    str += "</div>"
  }

  return str;
}

function updateHints(spot) {
  var n = grid[spot] - 1;
  for (var i = 0; i < 9; i++) {
    var row = Math.floor(spot / 9);
    var col = spot % 9;
    var begin = Math.floor(spot / 27) * 27 + Math.floor(spot % 9 / 3) * 3;
    
    for (var j = 0; j < 9; j++) {
      hints[row * 9 + j][n] = 0;
    }
    
    for (var j = 0; j < 9; j++) {
      hints[i * 9 + col][n] = 0;
    }
    
    for (var j = 0; j < 3; j++) {
      for (var k = 0; k < 3; k++) {
        hints[begin + j * 9 + k][n] = 0;
      }
    }
  }
}

function printOrder() {
  for (var i = 0; i < order.length; i++) {
    console.log(i + ": grid[" + order[i] + "] = " + grid[order[i]]);
  } 
}

function colorBorders() {
  var css = "#405952 2px solid";
  for (var row = 0; row < 9; row++) {
    for (var col = 0; col < 9; col++) {
      var id = "#" + (row * 9 + col).toString();
      if (row % 3 === 0) {
        $(id).css("border-top", css);
      }
      if (row % 3 === 2) {
        $(id).css("border-bottom", css);
      }
      if (col % 3 === 0) {
        $(id).css("border-left", css);
      }
      if (col % 3 === 2) {
        $(id).css("border-right", css);
      }
    }
  }
}

function generateSolution() {
  emptyGrid();
  getHints();
  generateGrid();
  while (!isFull()) {
    emptyGrid();
    getHints();
    generateGrid();
  }
  printGrid();
}

function backTrack() {
  var next;
  if (isFull()) {
    return true;
  }
  next = getNext();
  for (var i = 1; i <= 9; i++) {
    if (isSafe(grid[next], i)) {
      grid[next] = i;
    }
    if (backTrack()) {
      return true;
    } else {
      grid[next] = 0;
    }
  }
  return false;
}

function getNext() {
  for (var i = 0; i < grid.length(); i++) {
    if (grid[i] === 0) {
      return i;
    }
  }
  return -1;
}
function createPuzzle(clues) {
  var remove = 81 - clues;
  var indices = [];
  
  getHints();
  
  for (var i = 0; i < 81; i++) {
    indices.push(i);
  }
  
  for (var i = 0; i < remove; i++) {
    var index = Math.floor(Math.random() * indices.length);
    grid[indices[index]] = 0;
    indices.splice(index, 1);
  }
  
  for (var i = 0; i < 81; i++) {
    puzzle[i] = grid[i] !== 0 ? grid[i] : 0;
  }
}

function activateButton() {
  $("#one").css("background-color", "#f54f29");
  $("#one").css("color", "#ffd393");
  $("#one").css("border-color", "#ffd393");
}

$(document).ready(function() {
  generateSolution();
  createPuzzle(32);
  printGrid();
  activateButton();
});

$(document).on("click", ".number", function() {
  var old = active;
  active = $(this).attr("id");
  
  if (active === old) {
    mode = mode === "pick" ? "hint" : "pick";
  } else {
    var str = "#" + old;
    var bg = mode === "pick" ? "#f54f29" : "#405952";
    $(str).css("background-color", "#ffd393");
    $(str).css("color", "#405952");
  }
  var bg = mode === "pick" ? "#f54f29" : "#405952";
  $(this).css("background-color", bg);
  $(this).css("color", "#ffd393");
  $(this).css("border-color", "#ffd393");
});

$(document).on("click", ".square", function() {
  var value;
  var n = Number($(this).attr("id"));
  
  if (puzzle[n] === 0) {
    switch (active) {
      case "one"  : value = 1;
                    break;
      case "two"  : value = 2;
                    break;
      case "three": value = 3;
                    break;
      case "four" : value = 4;
                    break;
      case "five" : value = 5;
                    break;
      case "six"  : value = 6;
                    break;
      case "seven": value = 7;
                    break;
      case "eight": value = 8;
                    break;
      case "nine" : value = 9;
                    break;
    } // end of switch
    if (mode === "pick") {
      grid[n] = grid[n] === value ? 0 : value;
    } else {  // "hint" mode
      
      hints[n][value-1] = hints[n][value-1] === value ? 0 : value;
    }
    printGrid();
  }
});
!
999px

Console