IOS中如何数组内容比较得到最大值

有数组charAry,数组内容为:9, 87bh, j65, 159, 27, 3582, 64957, hg75b, 20140402b,请问怎么比较得到整数最大值64957

NSNumber * max = [charAry valueForKeyPath:@"@max.self"];