leetcode题解-53. Maximum Subarray && 448. Find All Numbers Disappeared in an Array

题目：

Find the contiguous subarray within an array (containing at least one number) which has the largest sum.

For example, given the array [-2,1,-3,4,-1,2,1,-5,4],
the contiguous subarray [4,-1,2,1] has the largest sum = 6.

本题就是寻找和最大的子串，我们的思路是遍历数组求和，一旦和小于零就让sum重置，并使用max记录最大和。代码如下所示：

public int maxSubArray(int[] A) {
int max = Integer.MIN_VALUE, sum = 0;
for (int i = 0; i < A.length; i++) {
if (sum < 0)
sum = A[i];
else
sum += A[i];
if (sum > max)
max = sum;
}
return max;
}

Find All Numbers Disappeared in an Array。 题目：

Given an array of integers where 1 ≤ a[i] ≤ n (n = size of array), some elements appear twice and others appear once.

Find all the elements of [1, n] inclusive that do not appear in this array.

Could you do it without extra space and in O(n) runtime? You may assume the returned list does not count as extra space.

Example:

Input:
[4,3,2,7,8,2,3,1]

Output:
[5,6]

本题是寻找长度为n的数组中丢失的数字。我们的思路是首先遍历数组，将元素对应的索引置为其负数，这样没有出现的数字对应的索引位置就是正数。代码如下所示：

public static List<Integer> findDisappearedNumbers1(int[] nums) {
List<Integer> ret = new ArrayList<Integer>();

for(int i = 0; i < nums.length; i++) {
int val = Math.abs(nums[i]) - 1;
if(nums[val] > 0) {
nums[val] = -nums[val];
}
}

for(int i = 0; i < nums.length; i++) {
if(nums[i] > 0) {
ret.add(i+1);
}
}
return ret;
}