Update 0042.接雨水.md

Author: Ainevsia

problems/0042.接雨水.md

@@ -926,6 +926,56 @@ int trap(int* height, int heightSize) {
 * 空间复杂度 O(1)
 
 
+Rust
+
+双指针
+
+```rust
+impl Solution {
+    pub fn trap(height: Vec<i32>) -> i32 {
+        let n = height.len();
+        let mut max_left = vec![0; height.len()];
+        let mut max_right = vec![0; height.len()];
+        max_left.iter_mut().zip(max_right.iter_mut().rev()).enumerate().fold((0, 0), |(lm, rm), (idx, (x, y))| {
+            let lmax = lm.max(height[idx]);
+            let rmax = rm.max(height[n - 1 - idx]);
+            *x = lmax; *y = rmax;
+            (lmax, rmax)
+        });
+        height.iter().enumerate().fold(0, |acc, (idx, x)| {
+            let h = max_left[idx].min(max_right[idx]);
+            if h > 0 { h - x + acc } else { acc }
+        })
+    }
+}
+```
+
+单调栈
+
+```rust
+impl Solution {
+    pub fn trap(height: Vec<i32>) -> i32 {
+        let mut stack = vec![];
+        let mut ans = 0;
+        for (right_pos, &right_h) in height.iter().enumerate() {
+            while !stack.is_empty() && height[*stack.last().unwrap()] <= right_h {
+                let mid_pos = stack.pop().unwrap();
+                if !stack.is_empty() {
+                    let left_pos = *stack.last().unwrap();
+                    let left_h = height[left_pos];
+                    let top = std::cmp::min(left_h, right_h);
+                    if top > height[mid_pos] {
+                        ans += (top - height[mid_pos]) * (right_pos - left_pos - 1) as i32;
+                    }
+                }
+            }
+            stack.push(right_pos);
+        }
+        ans
+    }
+}
+```
+
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