leetcode整理

Posted 2021-02-28 まだこの世界は

双数有序找单值

public int singleNonDuplicate(int[] nums) {
    int l = 0, h = nums.length - 1;
    while (l < h) {
        int m = l + (h - l) / 2;
        if (m % 2 == 1) {
            m--;   // 保证 l/h/m 都在偶数位，使得查找区间大小一直都是奇数
        }
        if (nums[m] == nums[m + 1]) {
            l = m + 2;
        } else {
            h = m;
        }
    }
    return nums[l];
}

不同的二叉搜索树 II

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

private List<TreeNode> generateSubtrees(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 = generateSubtrees(s, i - 1);
        List<TreeNode> rightSubtrees = generateSubtrees(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;
}

给表达式加括号(给定一个含有数字和运算符的字符串，为表达式添加括号，改变其运算优先级以求出不同的结果。)

public List<Integer> diffWaysToCompute(String input) {
    List<Integer> ways = new ArrayList<>();
    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 (int l : left) {
                for (int 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;
}

　　查找（dfs，bfs）