282:给表达式添加运算符

src/main/java/leetcode/editor/cn/ExpressionAddOperators.java

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+//给定一个仅包含数字 0-9 的字符串 num 和一个目标值整数 target ，在 num 的数字之间添加 二元 运算符（不是一元）+、- 或 * ，返回所
+//有能够得到目标值的表达式。 
+//
+// 
+//
+// 示例 1: 
+//
+// 
+//输入: num = "123", target = 6
+//输出: ["1+2+3", "1*2*3"] 
+// 
+//
+// 示例 2: 
+//
+// 
+//输入: num = "232", target = 8
+//输出: ["2*3+2", "2+3*2"] 
+//
+// 示例 3: 
+//
+// 
+//输入: num = "105", target = 5
+//输出: ["1*0+5","10-5"] 
+//
+// 示例 4: 
+//
+// 
+//输入: num = "00", target = 0
+//输出: ["0+0", "0-0", "0*0"]
+// 
+//
+// 示例 5: 
+//
+// 
+//输入: num = "3456237490", target = 9191
+//输出: [] 
+//
+// 
+//
+// 提示： 
+//
+// 
+// 1 <= num.length <= 10 
+// num 仅含数字 
+// -2³¹ <= target <= 2³¹ - 1 
+// 
+// Related Topics 数学 字符串 回溯 👍 324 👎 0
+
+package leetcode.editor.cn;
+
+import java.util.ArrayList;
+import java.util.List;
+
+//282:给表达式添加运算符
+class ExpressionAddOperators {
+    public static void main(String[] args) {
+        //测试代码
+        Solution solution = new ExpressionAddOperators().new Solution();
+    }
+
+    //力扣代码
+    //leetcode submit region begin(Prohibit modification and deletion)
+    class Solution {
+        int n;
+        String num;
+        int target;
+        List<String> ans;
+
+        public List<String> addOperators(String num, int target) {
+            this.n = num.length();
+            this.num = num;
+            this.target = target;
+            this.ans = new ArrayList<>();
+            StringBuffer expr = new StringBuffer();
+            backtrack(expr, 0, 0, 0);
+            return ans;
+        }
+
+        public void backtrack(StringBuffer expr, int i, long res, long mul) {
+            if (i == n) {
+                if (res == target) {
+                    ans.add(expr.toString());
+                }
+                return;
+            }
+            int signIndex = expr.length();
+            if (i > 0) {
+                expr.append(0);
+            }
+            long val = 0;
+            for (int j = i; j < n && (j == i || num.charAt(i) != '0'); ++j) {
+                val = val * 10 + num.charAt(j) - '0';
+                expr.append(num.charAt(j));
+                if (i == 0) {
+                    backtrack(expr, j + 1, val, val);
+                } else {
+                    expr.setCharAt(signIndex, '+');
+                    backtrack(expr, j + 1, res + val, val);
+                    expr.setCharAt(signIndex, '-');
+                    backtrack(expr, j + 1, res - val, -val);
+                    expr.setCharAt(signIndex, '*');
+                    backtrack(expr, j + 1, res - mul + mul * val, mul * val);
+                }
+            }
+            expr.setLength(signIndex);
+        }
+    }
+//leetcode submit region end(Prohibit modification and deletion)
+
+}

src/main/java/leetcode/editor/cn/doc/content/ExpressionAddOperators.md

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+<p>给定一个仅包含数字&nbsp;<code>0-9</code>&nbsp;的字符串 <code>num</code> 和一个目标值整数 <code>target</code> ，在 <code>num</code> 的数字之间添加 <strong>二元 </strong>运算符（不是一元）<code>+</code>、<code>-</code>&nbsp;或&nbsp;<code>*</code>&nbsp;，返回所有能够得到目标值的表达式。</p>
+
+<p>&nbsp;</p>
+
+<p><strong>示例 1:</strong></p>
+
+<pre>
+<strong>输入:</strong> <code>num = </code>"123", target = 6
+<strong>输出: </strong>["1+2+3", "1*2*3"] 
+</pre>
+
+<p><strong>示例&nbsp;2:</strong></p>
+
+<pre>
+<strong>输入:</strong> <code>num = </code>"232", target = 8
+<strong>输出: </strong>["2*3+2", "2+3*2"]</pre>
+
+<p><strong>示例 3:</strong></p>
+
+<pre>
+<strong>输入:</strong> <code>num = </code>"105", target = 5
+<strong>输出: </strong>["1*0+5","10-5"]</pre>
+
+<p><strong>示例&nbsp;4:</strong></p>
+
+<pre>
+<strong>输入:</strong> <code>num = </code>"00", target = 0
+<strong>输出: </strong>["0+0", "0-0", "0*0"]
+</pre>
+
+<p><strong>示例 5:</strong></p>
+
+<pre>
+<strong>输入:</strong> <code>num = </code>"3456237490", target = 9191
+<strong>输出: </strong>[]</pre>
+
+<p>&nbsp;</p>
+
+<p><strong>提示：</strong></p>
+
+<ul>
+	<li><code>1 &lt;= num.length &lt;= 10</code></li>
+	<li><code>num</code> 仅含数字</li>
+	<li><code>-2<sup>31</sup> &lt;= target &lt;= 2<sup>31</sup> - 1</code></li>
+</ul>
+<div><div>Related Topics</div><div><li>数学</li><li>字符串</li><li>回溯</li></div></div><br><div><li>👍 324</li><li>👎 0</li></div>