feat: add 043

Author: Blankj

README.md

@@ -64,6 +64,7 @@
 |17|[Letter Combinations of a Phone Number][017]|String, Backtracking|
 |19|[Remove Nth Node From End of List][019]|Linked List, Two Pointers|
 |33|[Search in Rotated Sorted Array][033]|Arrays, Binary Search|
+|43|[Multiply Strings][043]|Math, String|
 |554|[Brick Wall][554]|Hash Table|
 
 
@@ -124,6 +125,7 @@
 [017]: https://github.com/Blankj/awesome-java-leetcode/blob/master/note/017/README.md
 [019]: https://github.com/Blankj/awesome-java-leetcode/blob/master/note/019/README.md
 [033]: https://github.com/Blankj/awesome-java-leetcode/blob/master/note/033/README.md
+[043]: https://github.com/Blankj/awesome-java-leetcode/blob/master/note/043/README.md
 [554]: https://github.com/Blankj/awesome-java-leetcode/blob/master/note/554/README.md
 
 [004]: https://github.com/Blankj/awesome-java-leetcode/blob/master/note/004/README.md

note/043/README.md

@@ -0,0 +1,58 @@
+# [Multiply Strings][title]
+
+## Description
+
+Given two non-negative integers `num1` and `num2` represented as strings, return the product of `num1` and `num2`.
+
+**Note:**
+
+1. The length of both `num1` and `num2` is < 110.
+2. Both `num1` and `num2` contains only digits `0-9`.
+3. Both `num1` and `num2` does not contain any leading zero.
+4. You **must not use any built-in BigInteger library** or **convert the inputs to integer** directly.
+
+**Tags:** Math, String
+
+
+## 思路
+
+题意是让你计算两个非负字符串的乘积，我们模拟小学数学的方式来做，一位一位模拟计算，再各位累加。
+
+```java
+class Solution {
+    public String multiply(String num1, String num2) {
+        if (num1.equals("0") || num2.equals("0")) return "0";
+        int l1 = num1.length(), l2 = num2.length(), l = l1 + l2;
+        char[] ans = new char[l];
+        char[] c1 = num1.toCharArray();
+        char[] c2 = num2.toCharArray();
+        for (int i = l1 - 1; i >= 0; --i) {
+            int c = c1[i] - '0';
+            for (int j = l2 - 1; j >= 0; --j) {
+                ans[i + j + 1] +=  c * (c2[j] - '0');
+            }
+        }
+        for (int i = l - 1; i > 0; ++i) {
+            if (ans[i] > 9) {
+                ans[i - 1] += ans[i] / 10;
+                ans[i] %= 10;        
+            }
+         }
+        StringBuilder sb = new StringBuilder();
+        int i = 0;
+        for (; ; ++i) if (ans[i] != 0) break;
+        for (; i < ans.length; ++i) sb.append((char) (ans[i] + '0'));
+        return sb.toString();
+    }
+}
+```
+
+
+## 结语
+
+如果你同我一样热爱数据结构、算法、LeetCode，可以关注我GitHub上的LeetCode题解：[awesome-java-leetcode][ajl]
+
+
+
+[title]: https://leetcode.com/problems/multiply-strings
+[ajl]: https://github.com/Blankj/awesome-java-leetcode

src/com/blankj/medium/_043/Solution.java

@@ -0,0 +1,41 @@
+package com.blankj.medium._043;
+
+/**
+ * <pre>
+ *     author: Blankj
+ *     blog  : http://blankj.com
+ *     time  : 2017/10/17
+ *     desc  :
+ * </pre>
+ */
+public class Solution {
+    public String multiply(String num1, String num2) {
+        if (num1.equals("0") || num2.equals("0")) return "0";
+        int l1 = num1.length(), l2 = num2.length(), l = l1 + l2;
+        char[] ans = new char[l];
+        char[] c1 = num1.toCharArray();
+        char[] c2 = num2.toCharArray();
+        for (int i = l1 - 1; i >= 0; --i) {
+            int c = c1[i] - '0';
+            for (int j = l2 - 1; j >= 0; --j) {
+                ans[i + j + 1] +=  c * (c2[j] - '0');
+            }
+        }
+        for (int i = l - 1; i > 0; --i) {
+            if (ans[i] > 9) {
+                ans[i - 1] += ans[i] / 10;
+                ans[i] %= 10;
+            }
+        }
+        StringBuilder sb = new StringBuilder();
+        int i = 0;
+        for (; ; ++i) if (ans[i] != 0) break;
+        for (; i < ans.length; ++i) sb.append((char) (ans[i] + '0'));
+        return sb.toString();
+    }
+
+    public static void main(String[] args) {
+        Solution solution = new Solution();
+        System.out.println(solution.multiply("132", "19"));
+    }
+}