LeetCode第五天

leetcode 第五天

2018年1月6日

22.(566) Reshape the Matrix

JAVA

class Solution {
    public int[][] matrixReshape(int[][] nums, int r, int c) {
        int[][] newNums = new int[r][c];
        int size = nums.length*nums[0].length;
        if(r*c != size)
            return nums;
        for(int i=0;i<size;i++){
            newNums[i/c][i%c] = nums[i/nums[0].length][i%nums[0].length];
        }
        return newNums;
    }
}

23.(268) Missing Number

JAVA

class Solution {
    /*数列求和思想*/
    public int missingNumber(int[] nums) {
        int expectSum = nums.length*(nums.length+1)/2;
        int actualSum = 0;
        for(int num : nums) actualSum += num;
        return expectSum - actualSum;
    }
}

24.(243) Shortest Word Distance

JAVA

class Solution {
    public int shortestDistance(String[] words,String word1,String word2) {
        int idx1 = -1,idx2 = -1;
        int minDistance = words.length;
        int currentDistance;
        for(int i =0;i<words.length;i++){
            if(words[i].equals(word1))
                idx1=i;
            else if(words[i].equals(word2))
                idx2=i;
            
            if(idx1 != -1 && idx2 != -1){
                minDistance = Math.min(minDistance,Math.abs(idx1-idx2));
            }
        }
        return minDistance;
    }
}

25.(561) Array Partition I

JAVA

class Solution {
    public int arrayPairSum(int[] nums) {
        Arrays.sort(nums);
        int minSum = 0;
        for(int i = 0;i<nums.length;i+=2){
            minSum += Math.min(nums[i],nums[i+1]);
        }
        return minSum;
    }
}

26.(746) Min Cost Climbing Stairs

新知识点：动态规划（有点难度）

JAVA

class Solution {
    public int minCostClimbingStairs(int[] cost) {
        int f1=0;
        int f2=0;
        int f3=0;//f3为到达每一个楼层所需要花费的最小钱数(此楼层花费不算)
        for(int i = 2;i <= cost.length;i++){
            f3 = Math.min(f1+cost[i-2],f2+cost[i-1]);
            f1 = f2;
            f2 = f3;
        }
        return f3;
    }
}

27.(724) Find Pivot Index

JAVA

class Solution {
    public int pivotIndex(int[] nums) {
        int sum = 0,leftSum = 0;
        for(int num : nums) sum+=num;
        for(int i = 0;i < nums.length;i++){
            if(leftSum == sum - leftSum - nums[i])
                return i;
            leftSum += nums[i];
        }
        return -1;
        
    }
}

28.(66) Plus One

JAVA

class Solution {
    public int[] plusOne(int[] digits) {
        int n = digits.length;
        for(int i = n-1;i>=0;i--){
            if(digits[i]<9){
                digits[i]++;
                return digits;
            }
            digits[i] = 0;
        }
        //此处是为了防止原始数字为999...的情况
        int[] result = new int[n+1];
        result[0] = 1;
        return result;
    }
}

29.(1) Two Sum

注意Map/HashMap的声明、get()/containsKey()/put()等操作

JAVA

class Solution {
    public int[] twoSum(int[] nums, int target) {
        Map<Integer,Integer> map = new HashMap<Integer,Integer>();
        int[] result;
        for(int i = 0;i<nums.length;i++){
            if(map.containsKey(target - nums[i])){
                return new int[] {map.get(target-nums[i]),i};
            }else{
                map.put(nums[i],i);
            }
        }
        return null;
    }
}