review300: expression add operators

expressionAddOperators/review300.go

@@ -0,0 +1,55 @@
+package main
+
+import (
+	"strconv"
+	"strings"
+	"fmt"
+)
+func main() {
+
+	fmt.Println(addOperators("123456789", 45))
+
+}
+
+func addOperators(num string, target int) []string {
+
+	result := []string{}
+	if num == "" {
+		return result
+	}
+	next(num, 0, &result, nil, 0, 0, target)
+	return result
+}
+
+// start: n start index
+// p: "2+1"
+// pre: pre num, e.g. 2+11, pre num 11
+// cur: the result of curr expression, e.g. "2+1" == 3 == curr
+func next(num string, start int, result *[]string, p []string, pre, curr, target int) {
+	if start == len(num) {
+		if curr == target {
+			*result = append(*result, strings.Join(p, ""))
+		}
+		return
+	}
+
+	for end := start; end < len(num); end++ {
+		if end != start && num[start] == '0' {
+			return
+		}
+		istr := num[start : end+1]
+		n, _ := strconv.Atoi(istr)
+		if start == 0 {
+			next(num, end+1, result, []string{istr}, n, n, target)
+		} else {
+			tempPath := append(p, "+", istr)
+			next(num, end+1, result, tempPath, n, curr+n, target)
+			tempPath[len(p)] = "-"
+			next(num, end+1, result, tempPath, -n, curr-n, target)
+			tempPath[len(p)] = "*"
+			next(num, end+1, result, tempPath, n*pre, (curr-pre)+pre*n, target)
+		}
+
+	}
+
+}