区间DP:http://blog.csdn.net/lyhvoyage/article/details/24392533
数位dp: http://wenku.baidu.com/link?url=o3ER_gVCyB0qcKthM-Y8vPtAGZ_u5bzOu_gUCUhPcXC6YfaSDgtBSXNEEvvGvSzyuDE9TULcPNsDrRd9IUtQVHeKUVrnPUjyfWjCly_J7Xq
斜率 dp http://blog.csdn.net/azheng51714/article/details/8214165