LeetCode——分治

LeetCode类型题之分治

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目录

1. 给表达式加括号
2. 不同的二叉搜索树

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1. 给表达式加括号

public List<Integer> diffWaysToCompute(String input) {
        List<Integer> ways = new ArrayList<Integer>();
        for (int i = 0; i < input.length(); i++) {
            char c = input.charAt(i);
            if (c == '+' || c == '-' || c == '*') {
                List<Integer> left = diffWaysToCompute(input.substring(0, i));
                List<Integer> right = diffWaysToCompute(input.substring(i + 1));
                for (Integer l : left) {
                    for (Integer r : right) {
                        switch (c) {
                            case '+':
                                ways.add(l + r);
                                break;
                            case '-':
                                ways.add(l - r);
                                break;
                            case '*':
                                ways.add(l * r);
                                break;
                        }
                    }
                }
            }
        }
        if (ways.size() == 0) {
            ways.add(Integer.valueOf(input));
        }
        return ways;
    }

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2. 不同的二叉搜索树

public List<TreeNode> generateTrees(int n) {
        if (n < 1) {
            return new LinkedList<TreeNode>();
        }
        return generateTrees(1, n);
    }

    private List<TreeNode> generateTrees(int s, int e) {
        List<TreeNode> res = new LinkedList<TreeNode>();
        if (s > e) {
            res.add(null);
            return res;
        }
        for (int i = s; i <= e; i++) {
            List<TreeNode> leftSubtrees = generateTrees(s, i - 1);
            List<TreeNode> rightSubtrees = generateTrees(i + 1, e);
            for (TreeNode left : leftSubtrees) {
                for (TreeNode right : rightSubtrees) {
                    TreeNode root = new TreeNode(i);
                    root.left = left;
                    root.right = right;
                    res.add(root);
                }
            }
        }
        return res;
    }