求教一个js问题

Question

代码：
<script type="text/javascript">
var x = [[1,2],[2,1],[3,1],[1,4],[2,5],[4,2],[3,4],[4,3],[6,2],[3,5],[4,6],[1,6],[6,1],[3,8],[4,7],[3,6],[4,5],[3,3],[0,6],[1,5]];
document.write(x+"<br/>");
for(var j=0 ; j<x.length; j++){
 
for( var i= j+1 ; i< x.length; i++){ 
if( x[j][0]=== x[i][0]){  
     x.splice(i,1);
}
}}document.write(x);
</script>
输出：
1,2,2,1,3,1,1,4,2,5,4,2,3,4,4,3,6,2,3,5,4,6,1,6,6,1,3,8,4,7,3,6,4,5,3,3,0,6,1,5  //原数组
1,2,2,1,3,1,4,2,6,2,4,5,0,6      //最终结果数组期望得到结果：1,2,2,1,3,1,4,2,6,2,0,6   //比最终结果数组少一个元素[4,5,]
问题：为什么[4,5,]无法被删除

Answer 1

<script type="text/javascript">
var x = [[1,2],[2,1],[3,1],[1,4],[2,5],[4,2],[3,4],[4,3],[6,2],[3,5],[4,6],[1,6],[6,1],[3,8],[4,7],[3,6],[4,5],[3,3],[0,6],[1,5]];
document.write(x+"<br/>");
for(var j=0 ; j<x.length; j++){
  for( var i= j+1 ; i< x.length; i++){ 
    if( x[j][0]=== x[i][0]){  
      x.splice(i,1);
        i--;
      }
  }
}document.write(x);
</script>

Answer 2

<script type="text/javascript">
var x = [[1,2],[2,1],[3,1],[1,4],[2,5],[4,2],[3,4],[4,3],[6,2],[3,5],[4,6],[1,6],[6,1],[3,8],[4,7],[3,6],[4,5],[3,3],[0,6],[1,5]];
var arr=[];
var flag=false;
document.write(x+"<br/>");
for(var j=0 ; j<x.length; j++){
   arr[j]=[];
   for(var i=0;i<arr.length;i++){
     if(arr[i][0]===x[j][0]){
       flag=true;
    }
  if(!flag){
   arr[j].push(x[j]);
  }
}
}
document.write(arr.join(",").replace(/\s/g,""));
</script>