Here is a array:

```
2011-1-1
2011-1-2
2011-3-3
2012-1-4
2012-1-5
2012-6-7
2012-6-9
2013-1-3
2013-1-8
2013-9-1
2013-9-2
2014-3-7
2014-3-13
2014-4-1
2014-4-17

```

I need convert this `NSDictionary` with nested `NSArray` to json like this（use objective-c）:

```
  {
        "y2011m1": [
            "2011-1-1",
            "2011-1-2"
        ],
        "y2011m3": [
            "2011-3-3"
        ],
        "y2012m1": [
            "2012-1-4",
            "2012-1-5"
        ],
        "y2012m6": [
            "2012-6-7",
            "2012-6-9"
        ],
        "y2013m1": [
            "2013-1-3",
            "2013-1-8"
        ],
        "y2013m9": [
            "2013-9-1",
            "2013-6-2"
        ],
        "y2014m3": [
            "2014-3-7",
            "2014-3-13"
        ],
        "y2014m4": [
            "2014-4-1",
            "2014-4-17"
        ]
    }

```

The same years and same month become "yxxxmx",and group by it，i don't know how to do （use objective-c），please help me ，thanks.