算法题

接雨水

截屏2022-07-13 10.30.46.png

动态规划：找到当前位置，min（max左边，max右边），与当前位置计算差值可以得到当前位置接的雨水。

class Solution {
    public int trap(int[] height) {
        if (height == null || height.length == 0) return 0;

        int lastIdx = height.length - 2;

        int[] rightMaxes = new int[height.length];
        for (int i = lastIdx; i >= 1; i--) {
            rightMaxes[i] = Math.max(rightMaxes[i + 1], height[i + 1]);
        }

        // 遍历每一根柱子，看看每一根柱子上能放多少水
        int water = 0, leftMax = 0;
        for (int i = 1; i <= lastIdx; i++) {
            leftMax = Math.max(leftMax, height[i - 1]);
            // 求出左边最大、右边最大中的较小者
            int min = Math.min(leftMax, rightMaxes[i]);
            // 说明这根柱子不能放水
            if (min <= height[i]) continue;
            // 说明这根柱子能放水
            water += min - height[i];
        }

        return water;
    }
}

优化： 可以通过求得当前遍历到元素第二小的数值

  func trap(_ height: [Int]) -> Int {
        if height.isEmpty {
            return 0
        }

        var l = 0, r = height.count - 1, water = 0, lowMax = 0
        while (l < r) {
            var lower: Int 
            if (height[l] <= height[r]) {
                lower = height[l]
                l += 1
            } else {
                lower = height[r]
                r -= 1
            }
            // 抛去当前元素后，第二大的数值
            lowMax = max(lowMax, lower)
            water += lowMax - lower
        }
        return water
    }

盛水最多的容器

盛最多水的容器

思路 双指针，height比较小的元素向中间靠拢

func maxArea(_ height: [Int]) -> Int {
        if height.isEmpty {
            return 0
        }
        
        var l = 0, r = height.count - 1, water = 0
        
        while l < r {
            if height[l] <= height[r] {
                let minH = height[l]
                water = max(water, (r - l) * minH)
                while (l < r && height[l] <= minH) {
                    l += 1
                }
            } else {
                let minH = height[r]
                water = max(water, (r - l) * minH)
                while(l < r && height[r] <= minH) {
                    r -= 1
                }
            }
            
        }
        
        return water
    
    }

会议室II

截屏2022-07-13 20.43.30.png

分开排序

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        
        let arrays = [[0, 30], [5, 10], [15, 20]]
        print(minMeetingRooms(intervals: arrays))

        let arrays1 = [[7, 10], [2, 4]]
        print(minMeetingRooms(intervals: arrays1))
    }
    
    func minMeetingRooms(intervals: [[Int]]) -> Int {
        if intervals.isEmpty {
            return 0
        }
        var begins: [Int] = []
        var ends: [Int] = []
        for i in 0 ..< intervals.count {
            begins.append(intervals[i][0])
            ends.append(intervals[i][1])
        }
        
        
        begins.sort(by: <)
        ends.sort(by: <)
        
        var room = 0, endIndex = 0
        for begin in begins {
            if begin >= ends[endIndex] {
                endIndex += 1
            } else {
                room += 1
            }
        }
        return room
    }
}

合并两个有序数组

合并两个有序数组

func merge(_ nums1: inout [Int], _ m: Int, _ nums2: [Int], _ n: Int) {
        var i1 = m - 1, i2 = n - 1, cur = m + n - 1
        while i2 >= 0 {
            if i1 >= 0 && nums2[i2] < nums1[i1] {
                nums1[cur] = nums1[i1]
                cur -= 1
                i1 -= 1
            } else {
                nums1[cur] = nums2[i2]
                cur -= 1
                i2 -= 1
            }
        }
    }

Z 字形变换

Z字形变换

思路：关键点在于理解，将加入方向调转

 func convert(_ s: String, _ numRows: Int) -> String {
        
        if numRows == 1 {
            return s
        }
        
        var arrays = [String](repeating: "", count:numRows)
        
        
        var currentRow = 0
        var goingDown = false
        
        for character in s {
            arrays[currentRow].append(character)
            
            if currentRow == numRows - 1 || currentRow == 0 {
                goingDown = !goingDown
            }
            // 关键点就是这里的翻转，从上到下后改为从下到上
            currentRow += goingDown ? 1 : -1
        }
        
        return arrays.joined()
    }