LeetCode第六天

第六天

30.(219) Contains Duplicate II

JAVA

class Solution {
    public boolean containsNearbyDuplicate(int[] nums, int k) {
        Map<Integer,Integer> map = new HashMap<Integer,Integer>();
        for(int i =0;i<nums.length;i++){
            if(map.containsKey(nums[i]))
                if(i-map.get(nums[i])<=k)
                    return true;
                else
                    map.put(nums[i],i);
            else
                map.put(nums[i],i);
        }
        return false;
    }
}

31.(747) Largest Number At Least Twice of Others

JAVA

class Solution {
    public int dominantIndex(int[] nums) {
        int maxIndex = 0;
        for(int i =0;i<nums.length;i++){
            if(nums[i]>nums[maxIndex])
                maxIndex = i;
        }

        for(int i = 0 ;i<nums.length;i++){
            if(i!=maxIndex&&nums[maxIndex]<nums[i]*2)
                return -1;
        }
        return maxIndex;
    }
}

32.(665) Non-decreasing Array

有点难度

JAVA

class Solution {
    public boolean checkPossibility(int[] nums) {
        boolean isFirst = true;
        for(int i = 0 ;i < nums.length-1;i++){
            if(nums[i+1] < nums[i]){
                if(isFirst){
                    if(i == 0){//首位置为较小值
                        nums[i] = nums[i+1];
                    }else if(i == nums.length -2){//末尾置为大值
                        nums[i+1] = nums[i];
                    }else{//中间位置通过二者左右两边数的值，判断把哪位置为何值。
                        if(nums[i]<nums[i+2]){
                            nums[i+1] = nums[i];
                        }else if(nums[i+1]>nums[i-1]){
                            nums[i] = nums[i+1];
                        }else{
                            return false;
                        }
                    }
                    isFirst = false;
                }else{
                    return false;
                }
        }
        }
        return true;
    }
}

33.(532) K-diff Pairs in an Array

JAVA

//注意，这里的Map中存放的是 （num,num+k）or(num,null)
//之后计算Map中value有多少个非Null即可；
class Solution {
    public int findPairs(int[] nums, int k) {
        int pairs = 0;
        Map<Integer,Integer> map = new HashMap<Integer,Integer>();
        if(nums==null||nums.length == 0||k<0) return 0;
        for (int i = 0 ;i<nums.length;i++){
            if(map.containsKey(nums[i]-k))
                map.put(nums[i]-k,nums[i]);
            if(map.containsKey(nums[i]+k))
                map.put(nums[i],nums[i]+k);
            if(!map.containsKey(nums[i]))
                map.put(nums[i],null);
        }

        for(Integer key : map.keySet()) {
             if(map.get(key)!=null)
                 pairs++;
        }
        return pairs;
    }
}

34.(189) Rotate Array

JAVA

class Solution {
    public void rotate(int[] nums, int k) {
        k %=nums.length;
        reverse(nums,0,nums.length-1);
        reverse(nums,0,k-1);
        reverse(nums,k,nums.length-1);
    }
    public void reverse(int[] nums,int start,int end){
        while(start<end){
            int temp = nums[start];
            nums[start] = nums[end];
            nums[end] = temp;
            start++;
            end--;
        }
    }
}

35.(169) Majority Element

JAVA

class Solution {
    public int majorityElement(int[] nums) {
        int count = 0;
        Integer candidate = null;
        for(int num :nums){
            if(count == 0)
                candidate = num;
            count += (num == candidate)?1:-1;

        }
        return candidate;
    }
}

36.(167) Two Sum II - Input array is sorted

JAVA

class Solution {
    public int[] twoSum(int[] numbers, int target) {
        int start = 0;
        int end = numbers.length-1;
        while(start<end && numbers[start]+numbers[end] != target){
            if(numbers[start]+numbers[end]>target)
                end--;
            else
                start++;
        }
        return new int[] {start+1,end+1};
    }
}

37.(661) Image Smoother

JAVA

class Solution {
    public int[][] imageSmoother(int[][] M) {
        int R = M.length,C = M[0].length;
        int[][] result = new int[R][C];
        for(int r = 0 ;r<R;r++)
            for(int c = 0;c<C;c++){
                int count = 0;
                result[r][c] = 0;
                for(int nr=r-1;nr <= r+1;nr++)
                    for(int nc = c-1;nc <= c+1;nc++)
                        if(nr>=0&&nr<R&&nc>=0&&nc<C){
                            count++;
                            result[r][c] +=M[nr][nc];
                        }
                result[r][c] /= count;
            }
        return result;
    }
}

38.(53) Maximum Subarray

动态优化问题！重点，跟上楼梯那道题类似

解题思路

JAVA

class Solution {
    public int maxSubArray(int[] nums) {
        int n = nums.length;
        int[] DP = new int[n];
        DP[0] = nums[0];
        int maxSum = DP[0];
        for(int i = 1;i<nums.length;i++){
            DP[i] = nums[i]+(DP[i-1]>0?DP[i-1]:0);
            maxSum = Math.max(maxSum,DP[i]);
        }
        return maxSum;
    }
}

39.(697) Degree of an Array

JAVA

class Solution {
    public int findShortestSubArray(int[] nums) {
        Map<Integer,Integer> left = new HashMap(),right = new HashMap(),count = new HashMap();
        for(int i = 0;i<nums.length;i++){
            if(left.get(nums[i]) == null) left.put(nums[i],i);
            right.put(nums[i],i);
            count.put(nums[i],count.getOrDefault(nums[i],0)+1);
        }
        int degree = Collections.max(count.values());//找出count中value的最大值
        int minLength = nums.length;
        for(int key : count.keySet()){
            if(count.get(key)==degree){
                minLength = Math.min(minLength,right.get(key)-left.get(key)+1);
            }
        }
        return minLength;
    }
}