uniquePaths f[i][j] = f[i-1][j] + f[i][j-1]
Unique Paths II
Minimum Path Sum
Merge Two Sorted Lists 合并两个排序链表
Add Binary 二进制数相加 翻转 相加 再翻转
Plus One  大整数+1
sqrt x[i]=(x[i-1]+n/x[i-1])/2
Climbing Stairs  菲波拉契数列