1. A method of classifying a user input, comprising: receiving user input of Roman characters from one or more users; identifying, by one or more computers, a first ambiguous word in the user input, the first ambiguous word being identified as a string of the Roman characters that is both a non-pinyin word written in the Roman characters and a valid pinyin input of one or more pinyin syllables that correspond to an entry in a Chinese dictionary; determining, by one or more computers, that each of one or more words adjacent to the first ambiguous word, are classified as belonging to one of a pinyin context or a non-pinyin context; classifying, by one or more computers, the first ambiguous word as one of a pinyin input or a non-pinyin word, the classification of the first ambiguous word being based on the classification of the one or more words adjacent to the ambiguous word, the first ambiguous word being classified as a pinyin input in response to the one or more words adjacent to the ambiguous word being classified as belonging to the pinyin context, and the first ambiguous word being classified as a non-pinyin word in response to the one or more words adjacent to the first ambiguous word being classified as belonging to the non-pinyin context; identifying a second ambiguous word in the user input; determining that the second ambiguous word is located between a first word that is classified as belonging to the pinyin context and a second word that is classified as belonging to the non-pinyin context; determining a probability that the second ambiguous word corresponds to an entry in a Chinese dictionary; and classifying the second ambiguous word based on the probability, the second ambiguous word being classified as a pinyin input when the probability meets a threshold probability.