Expression Add Operators

最新推荐文章于 2020-11-05 00:11:14 发布

原创最新推荐文章于 2020-11-05 00:11:14 发布 · 260 阅读

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本文介绍了一种通过递归回溯的方法实现字符串操作表达式的求值算法。该算法能够处理加减乘三种运算，并确保当输入数字串与目标值时，能返回所有可能的有效数学表达式组合。

public class Solution {
    public List<String> addOperators(String num, int target) {
        List<String> res = new LinkedList<>();
         helper(res, num, target, "", 0, 0);
         return res;
    }
    
    private void helper(List<String> res, String num, int target, String temp, long curRes, long prevNum) {
        if (curRes == target && num.length() == 0) {
            res.add(new String(temp));
            return;
        }
        for (int i = 1; i <= num.length(); i++) {
            String curStr = num.substring(0, i);
            if (curStr.length() > 1 && curStr.charAt(0) == '0') {
                return;
            }
            long curNum = Long.valueOf(curStr);
            String nextNum = num.substring(i);
            if (temp.length() != 0) {
                helper(res, nextNum, target, temp + '*' + curStr, curRes - prevNum + prevNum * curNum, prevNum * curNum);
                helper(res, nextNum, target, temp + '+' + curStr, curRes + curNum, curNum);
                helper(res, nextNum, target, temp + '-' + curStr, curRes - curNum, -curNum);
            } else {
                helper(res, nextNum, target, curStr, curNum, curNum);
            }
        }
    }
}