docs/Leetcode_Solutions/179._Largest_Number.ipynb

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    "# 179.Largest Number 最大数\n",
    "\n",
    "### 难度：Medium\n",
    "\n",
    "## 刷题内容\n",
    "\n",
    "> 原题链接\n",
    "\n",
    " - 中文：https://leetcode-cn.com/problems/largest-number/description/\n",
    " - 英文：https://leetcode.com/problems/largest-number/\n",
    "\n",
    "> 内容描述\n",
    "\n",
    "```\n",
    "给定一组非负整数，重新排列它们的顺序使之组成一个最大的整数。\n",
    "\n",
    "示例 1:\n",
    "输入: [10,2]\n",
    "输出: 210\n",
    "\n",
    "示例 2:\n",
    "输入: [3,30,34,5,9]\n",
    "输出: 9534330\n",
    "说明: 输出结果可能非常大，所以你需要返回一个字符串而不是整数。\n",
    "```\n",
    "\n",
    "## 解题方案\n",
    "\n",
    "> 思路 1\n",
    "\n",
    " - 先排序，后合并，若最后为空字符串，则返回 '0'\n",
    "\n",
    "其中的排序思想是字符串的经典比较：\n",
    "\n",
    "【注】：在 Python3 中没有了 cmp，只有 key，我们可以使用 functools.cmp_to_key 去修饰一下。\n",
    "\n",
    "cmp 函数比较两个对象，如 (x, y)，若 x > y，则返回 1，if x == y，return 0，如果 x < y，则返回 -1 。\n",
    "\n",
    "下面这个是 python2 的解决方案，可以 AC ，但是在我们的 Python3 中就会报如下错误"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {},
   "outputs": [
    {
     "ename": "TypeError",
     "evalue": "'cmp' is an invalid keyword argument for this function",
     "output_type": "error",
     "traceback": [
      "\u001b[1;31m---------------------------------------------------------------------------\u001b[0m",
      "\u001b[1;31mTypeError\u001b[0m                                 Traceback (most recent call last)",
      "\u001b[1;32m<ipython-input-1-ac3558977848>\u001b[0m in \u001b[0;36m<module>\u001b[1;34m()\u001b[0m\n\u001b[0;32m     11\u001b[0m \u001b[0ms\u001b[0m \u001b[1;33m=\u001b[0m \u001b[0mSolution\u001b[0m\u001b[1;33m(\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0;32m     12\u001b[0m \u001b[0mnums\u001b[0m \u001b[1;33m=\u001b[0m \u001b[1;33m[\u001b[0m\u001b[1;36m10\u001b[0m\u001b[1;33m,\u001b[0m \u001b[1;36m2\u001b[0m\u001b[1;33m]\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[1;32m---> 13\u001b[1;33m \u001b[0mprint\u001b[0m\u001b[1;33m(\u001b[0m\u001b[0ms\u001b[0m\u001b[1;33m.\u001b[0m\u001b[0mlargestNumber\u001b[0m\u001b[1;33m(\u001b[0m\u001b[0mnums\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0m",
      "\u001b[1;32m<ipython-input-1-ac3558977848>\u001b[0m in \u001b[0;36mlargestNumber\u001b[1;34m(self, nums)\u001b[0m\n\u001b[0;32m      6\u001b[0m         \"\"\"\n\u001b[0;32m      7\u001b[0m         \u001b[0mnums\u001b[0m \u001b[1;33m=\u001b[0m \u001b[1;33m[\u001b[0m\u001b[0mstr\u001b[0m\u001b[1;33m(\u001b[0m\u001b[0mnum\u001b[0m\u001b[1;33m)\u001b[0m \u001b[1;32mfor\u001b[0m \u001b[0mnum\u001b[0m \u001b[1;32min\u001b[0m \u001b[0mnums\u001b[0m\u001b[1;33m]\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[1;32m----> 8\u001b[1;33m         \u001b[0mnums\u001b[0m\u001b[1;33m.\u001b[0m\u001b[0msort\u001b[0m\u001b[1;33m(\u001b[0m\u001b[0mcmp\u001b[0m\u001b[1;33m=\u001b[0m\u001b[1;32mlambda\u001b[0m \u001b[0mx\u001b[0m\u001b[1;33m,\u001b[0m \u001b[0my\u001b[0m\u001b[1;33m:\u001b[0m \u001b[0mcmp\u001b[0m\u001b[1;33m(\u001b[0m\u001b[0my\u001b[0m\u001b[1;33m+\u001b[0m\u001b[0mx\u001b[0m\u001b[1;33m,\u001b[0m \u001b[0mx\u001b[0m\u001b[1;33m+\u001b[0m\u001b[0my\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0m\u001b[0;32m      9\u001b[0m         \u001b[1;32mreturn\u001b[0m \u001b[1;34m''\u001b[0m\u001b[1;33m.\u001b[0m\u001b[0mjoin\u001b[0m\u001b[1;33m(\u001b[0m\u001b[0mnum\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m.\u001b[0m\u001b[0mlstrip\u001b[0m\u001b[1;33m(\u001b[0m\u001b[1;34m'0'\u001b[0m\u001b[1;33m)\u001b[0m \u001b[1;32mif\u001b[0m \u001b[1;34m''\u001b[0m\u001b[1;33m.\u001b[0m\u001b[0mjoin\u001b[0m\u001b[1;33m(\u001b[0m\u001b[0mnum\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m.\u001b[0m\u001b[0mlstrip\u001b[0m\u001b[1;33m(\u001b[0m\u001b[1;34m'0'\u001b[0m\u001b[1;33m)\u001b[0m \u001b[1;32melse\u001b[0m \u001b[1;34m'0'\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0;32m     10\u001b[0m \u001b[1;33m\u001b[0m\u001b[0m\n",
      "\u001b[1;31mTypeError\u001b[0m: 'cmp' is an invalid keyword argument for this function"
     ]
    }
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   "source": [
    "class Solution(object):\n",
    "    def largestNumber(self, nums):\n",
    "        \"\"\"\n",
    "        :type nums: List[int]\n",
    "        :rtype: str\n",
    "        \"\"\"\n",
    "        nums = [str(num) for num in nums]\n",
    "        nums.sort(cmp=lambda x, y: cmp(y+x, x+y))\n",
    "        return ''.join(num).lstrip('0') if ''.join(num).lstrip('0') else '0'\n",
    "    \n",
    "s = Solution()\n",
    "nums = [10, 2]\n",
    "print(s.largestNumber(nums))"
   ]
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   "source": [
    "或者更简单的写法，可以写成下面这样（Python2适用，python3 会报错）："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {},
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    {
     "ename": "TypeError",
     "evalue": "'cmp' is an invalid keyword argument for this function",
     "output_type": "error",
     "traceback": [
      "\u001b[1;31m---------------------------------------------------------------------------\u001b[0m",
      "\u001b[1;31mTypeError\u001b[0m                                 Traceback (most recent call last)",
      "\u001b[1;32m<ipython-input-2-af910a43d954>\u001b[0m in \u001b[0;36m<module>\u001b[1;34m()\u001b[0m\n\u001b[0;32m     11\u001b[0m \u001b[0ms\u001b[0m \u001b[1;33m=\u001b[0m \u001b[0mSolution\u001b[0m\u001b[1;33m(\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0;32m     12\u001b[0m \u001b[0mnums\u001b[0m \u001b[1;33m=\u001b[0m \u001b[1;33m[\u001b[0m\u001b[1;36m10\u001b[0m\u001b[1;33m,\u001b[0m \u001b[1;36m2\u001b[0m\u001b[1;33m]\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[1;32m---> 13\u001b[1;33m \u001b[0mprint\u001b[0m\u001b[1;33m(\u001b[0m\u001b[0ms\u001b[0m\u001b[1;33m.\u001b[0m\u001b[0mlargestNumber\u001b[0m\u001b[1;33m(\u001b[0m\u001b[0mnums\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0m",
      "\u001b[1;32m<ipython-input-2-af910a43d954>\u001b[0m in \u001b[0;36mlargestNumber\u001b[1;34m(self, nums)\u001b[0m\n\u001b[0;32m      6\u001b[0m         \"\"\"\n\u001b[0;32m      7\u001b[0m         \u001b[0mnums\u001b[0m \u001b[1;33m=\u001b[0m \u001b[1;33m[\u001b[0m\u001b[0mstr\u001b[0m\u001b[1;33m(\u001b[0m\u001b[0mnum\u001b[0m\u001b[1;33m)\u001b[0m \u001b[1;32mfor\u001b[0m \u001b[0mnum\u001b[0m \u001b[1;32min\u001b[0m \u001b[0mnums\u001b[0m\u001b[1;33m]\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[1;32m----> 8\u001b[1;33m         \u001b[0mnums\u001b[0m\u001b[1;33m.\u001b[0m\u001b[0msort\u001b[0m\u001b[1;33m(\u001b[0m\u001b[0mcmp\u001b[0m\u001b[1;33m=\u001b[0m\u001b[1;32mlambda\u001b[0m \u001b[0mx\u001b[0m\u001b[1;33m,\u001b[0m \u001b[0my\u001b[0m\u001b[1;33m:\u001b[0m \u001b[0mcmp\u001b[0m\u001b[1;33m(\u001b[0m\u001b[0my\u001b[0m\u001b[1;33m+\u001b[0m\u001b[0mx\u001b[0m\u001b[1;33m,\u001b[0m \u001b[0mx\u001b[0m\u001b[1;33m+\u001b[0m\u001b[0my\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0m\u001b[0;32m      9\u001b[0m         \u001b[1;32mreturn\u001b[0m \u001b[1;34m''\u001b[0m\u001b[1;33m.\u001b[0m\u001b[0mjoin\u001b[0m\u001b[1;33m(\u001b[0m\u001b[0mnum\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m.\u001b[0m\u001b[0mlstrip\u001b[0m\u001b[1;33m(\u001b[0m\u001b[1;34m'0'\u001b[0m\u001b[1;33m)\u001b[0m \u001b[1;32mor\u001b[0m \u001b[1;34m'0'\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0;32m     10\u001b[0m \u001b[1;33m\u001b[0m\u001b[0m\n",
      "\u001b[1;31mTypeError\u001b[0m: 'cmp' is an invalid keyword argument for this function"
     ]
    }
   ],
   "source": [
    "class Solution(object):\n",
    "    def largestNumber(self, nums):\n",
    "        \"\"\"\n",
    "        :type nums: List[int]\n",
    "        :rtype: str\n",
    "        \"\"\"\n",
    "        nums = [str(num) for num in nums]\n",
    "        nums.sort(cmp=lambda x, y: cmp(y+x, x+y))\n",
    "        return ''.join(num).lstrip('0') or '0'\n",
    "    \n",
    "s = Solution()\n",
    "nums = [10, 2]\n",
    "print(s.largestNumber(nums))"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "下面这个解法就是我们在 Python3 中的解法"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "210\n"
     ]
    }
   ],
   "source": [
    "from functools import cmp_to_key\n",
    "\n",
    "# 比较函数\n",
    "def compare(a, b):\n",
    "    return int(b + a) - int(a + b)\n",
    "\n",
    "class Solution(object):\n",
    "    def largestNumber(self, nums):\n",
    "        \"\"\"\n",
    "        :type nums: List[int]\n",
    "        :rtype: str\n",
    "        \"\"\"\n",
    "        nums = sorted([str(x) for x in nums], key=cmp_to_key(compare))\n",
    "        return str(int(''.join(nums)))\n",
    "    \n",
    "s = Solution()\n",
    "nums = [10, 2]\n",
    "print(s.largestNumber(nums))"
   ]
  }
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