输入框光标定位

function selectText  (el,start,end) {
  if(el.createTextRange){
        var Range = el.createTextRange();
        Range.collapse();
        Range.moveEnd('character',end);
        Range.moveStart('character',start);
        Range.select();
    };
    if(el.setSelectionRange){
        el.focus();
        el.setSelectionRange(start,end);  //设光标 
    };
}