算法修炼Day58|● 739. 每日温度 ● 496.下一个更大元素 I

// 超时
// 暴力解法：两层for循环，一层定义起始位置，一层定义终止位置。终止的情况有两种：一个是遇到大于当前节点的值，一个遍历到结尾没有遇到大于当前节点的值，使用一个局部变量和一个标识符标记即可
class Solution {
    public int[] dailyTemperatures(int[] temperatures) {
        int[] answer = new int[temperatures.length];
        for (int i = 0; i < temperatures.length; i++) {
            int res = 0;
            boolean flag = false;
            for (int j = i; j < temperatures.length; j++) {
                if (temperatures[j] > temperatures[i]) {
                    answer[i] = res;
                    flag = true;
                    break;
                }
                res++;
            }
            if (!flag) {
                answer[i] = 0;
            }
        }
        return answer;
    }
}
// 单调栈
class Solution {
    public int[] dailyTemperatures(int[] temperatures) {
        Stack<Integer> stack = new Stack<>(); // 存储数组索引
        stack.push(0);
        int[] res = new int[temperatures.length];
        for (int i = 1; i < temperatures.length; i++) {
            if (temperatures[i] <= temperatures[stack.peek()]) {
                stack.push(i);
            } else {
                while (!stack.isEmpty() && temperatures[i] > temperatures[stack.peek()]) {
                    res[stack.peek()] = i - stack.peek();
                    stack.pop();
                }
                stack.push(i); // 如果符合单调递增，则加入栈
            }
        }
        return res;
    }
}

// 单调栈
class Solution {
    public int[] nextGreaterElement(int[] nums1, int[] nums2) {
        Stack<Integer> stack = new Stack<>();
        int[] res = new int[nums1.length];
        Arrays.fill(res, -1);
        Map<Integer, Integer> map = new HashMap<>();
        for (int i = 0; i < nums1.length; i++) {
            map.put(nums1[i], i);
        }
        stack.add(0);
        for (int i = 1; i < nums2.length; i++) {
            if (nums2[i] <= stack.peek()) {
                stack.add(i);
            } else {
                while (!stack.isEmpty() && nums2[i] > nums2[stack.peek()]) {
                    if (map.containsKey(nums2[stack.peek()])) {
                        int idx = map.get(nums2[stack.peek()]);
                        res[idx] = nums2[i]; 
                    }
                    // stack.pop();
                }
                stack.add(i);
            }
        }
        return res;
    }
}
// 暴力解法：ac
class Solution {
    public int[] nextGreaterElement(int[] nums1, int[] nums2) {
        int[] res = new int[nums1.length];
        for (int i = 0; i < nums1.length; i++) {
            res[i] = -1;
        }
        for (int i = 0; i < nums1.length; i++) {
            for (int j = 0; j < nums2.length; j++) {
                if (nums1[i] == nums2[j]) {
                    for (int k = j + 1; k < nums2.length; k++) {
                        if (nums2[k] > nums2[j]) {
                            res[i] = nums2[k];
                            break;
                        }
                    }
                    break;
                }
            }
        }
        return res;
    }
}